diff --git a/maidroid/api.lua b/maidroid/api.lua
index 2d8ceef..b187dc4 100644
--- a/maidroid/api.lua
+++ b/maidroid/api.lua
@@ -30,6 +30,14 @@ function maidroid.is_core(item_name)
 	return false
 end
 
+-- maidroid.is_maidroid reports whether a name is maidroid's name.
+function maidroid.is_maidroid(name)
+	if maidroid.registered_maidroids[name] then
+		return true
+	end
+	return false
+end
+
 ---------------------------------------------------------------------
 
 -- maidroid.maidroid represents a table that contains common methods
@@ -117,15 +125,12 @@ end
 -- maidroid.maidroid.set_yaw_by_direction sets the maidroid's yaw
 -- by a direction vector.
 function maidroid.maidroid.set_yaw_by_direction(self, direction)
-	self.object:setyaw(math.atan2(direction.z, direction.x) + math.pi / 2)
+	self.object:setyaw(math.atan2(direction.z, direction.x) - math.pi / 2)
 end
 
 -- maidroid.maidroid.get_wield_item_info returns the maidroid's wield item's stack.
 function maidroid.maidroid.get_wield_item_stack(self)
 	local inv = self:get_inventory()
-	if inv:is_empty("wield_item") then
-		return nil
-	end
 	return inv:get_stack("wield_item", 1)
 end
 
@@ -196,6 +201,59 @@ minetest.register_on_player_receive_fields(function(player, formname, fields)
 	end
 end)
 
+--------------------------------------------------------------------
+
+-- register empty item entity definition.
+-- this entity may be hold by maidroid's hands.
+;(function ()
+	local function on_activate(self, staticdata)
+		-- attach to the nearest maidroid.
+		local all_objects = minetest.get_objects_inside_radius(self.object:getpos(), 0.1)
+		for _, obj in ipairs(all_objects) do
+			local luaentity = obj:get_luaentity()
+
+			if maidroid.is_maidroid(luaentity.name) then
+				self.object:set_attach(obj, "Arm_R", {x = 0.075, y = 0.60, z = -0.20}, {x = 0, y = 90, z = 0})
+				self.object:set_properties{textures="air"}
+				break
+			end
+		end
+	end
+
+	local function on_step(self, staticdata)
+		local all_objects = minetest.get_objects_inside_radius(self.object:getpos(), 0.1)
+		for _, obj in ipairs(all_objects) do
+			local luaentity = obj:get_luaentity()
+
+			if maidroid.is_maidroid(luaentity.name) then
+				local stack = luaentity:get_wield_item_stack()
+				if stack:get_name() ~= itemname then
+					if stack:is_empty() then
+						self.itemname = "air"
+						self.object:set_properties{textures=self.itemname}
+					else
+						self.itemname = stack:get_name()
+						self.object:set_properties{textures=self.itemname}
+					end
+				end
+				break
+			end
+		end
+	end
+
+	minetest.register_entity("maidroid:dummy_item", {
+		hp_max		= 1,
+		visual		= "wielditem",
+		visual_size	= {x = 0.025, y = 0.025},
+		collisionbox	= {0, 0, 0, 0, 0, 0},
+		physical	= false,
+		textures	= {"air"},
+		on_activate	= on_activate,
+		on_step         = on_step,
+		itemname        = "air",
+	})
+ end) ()
+
 ---------------------------------------------------------------------
 
 -- maidroid.register_core registers a definition of a new core.
@@ -211,6 +269,8 @@ end
 
 -- maidroid.register_maidroid registers a definition of a new maidroid.
 function maidroid.register_maidroid(product_name, def)
+	maidroid.registered_maidroids[product_name] = def
+	
 	-- initialize manufacturing number of a new maidroid.
 	if maidroid.manufacturing_data[product_name] == nil then
 		maidroid.manufacturing_data[product_name] = 0
@@ -310,6 +370,9 @@ function maidroid.register_maidroid(product_name, def)
 			maidroid.manufacturing_data[product_name] = maidroid.manufacturing_data[product_name] + 1
 			create_inventory(self)
 			self.nametag = self.inventory_name
+
+			-- attach dummy item to new maidroid.
+			minetest.add_entity(self.object:getpos(), "maidroid:dummy_item")
 		else
 			-- if static data is not empty string, this object has beed already created.
 			local data = minetest.deserialize(staticdata)
@@ -377,6 +440,7 @@ function maidroid.register_maidroid(product_name, def)
 
 	-- on_step is a callback function that is called every delta times.
 	local function on_step(self, dtime)
+		-- do core method.
 		if (not self.pause) and self.core_name ~= "" then
 			local core = maidroid.registered_cores[self.core_name]
 			core.on_step(self, dtime)
diff --git a/maidroid/models/maidroid.b3d b/maidroid/models/maidroid.b3d
index ca6662a..96cbc32 100644
Binary files a/maidroid/models/maidroid.b3d and b/maidroid/models/maidroid.b3d differ
diff --git a/maidroid/models/maidroid.blend b/maidroid/models/maidroid.blend
index 3a4b7ec..47add40 100644
Binary files a/maidroid/models/maidroid.blend and b/maidroid/models/maidroid.blend differ