To form a clearance between an omen-close cover and a main body once without instantaneously bringing an engaging pawl and an engaging pin to a locked state when the open-close cover is closed.

In this lock mechanism, a recording paper cover 20 is installed to a device case 11 in a manner that the cover 20 can be opened and closed, and the cover 20 is brought to a closed state to the device case 11 by the relationship of engagement between a first engaging pawl 25 and a first engaging pin 17 while being brought to an open state by releasing the relationship of engagement. An abutting section, with which the first engaging pin is abutted, is formed to the first engaging pawl 25, and the abutting section is abutted against the first engaging pin 17 once and a clearance is formed between the recording paper cover 20 and the device case 11 before the first engaging pawl 25 and the first engaging pin 17 are engaged. Accordingly, the recording paper cover 20 and the device case 11 do not collide.