PURPOSE: To provide a thermal transfer type image forming device having a high secrecy holding property, wherein a heat transfer printing material can be taken out only when a user ID is inputted.

CONSTITUTION: An inserting and detaching port 43 is provided in a side face of a main body of the device so that a heat transfer printing ribbon cassette can be fit to a holder and be drawn out therethrough, and is closed by a door body 44. Normally, a tip pawl part 51a of an engaging and stopping lever 51 is engaged to a hook 50 fitted to the back side of the door body 44 to lock the door body 44 so that the ribbon cassette can not be drawn out. When a user ID is inputted through an input means, a magnet 54 operates to release the engagement of the tip pawl part 51a with the hook 50, so that the door body 44 can be opened.