Possibility/impossibility of recording onto a recording medium accommodated in a cartridge is switched by moving a plug for preventing accidental deletion of information recorded on the recording medium along a bottom plate of a lower case. A projecting portion is formed at the plug. A flexible pin projects from an upper case so as to form a detent structure together with the projecting portion. At the time of joining together the upper and lower cases, even when the plug is positioned between a position which express possibility of recording and a position which express impossibility of recording, as the upper and lower cases are joined together, the pin relatively slides along an inclined surface formed at a top end of the projecting portion, and the plug moves to the position which makes recording possible. In this way, assemblability is good.