A locking device of a tray includes a button; a first hooking part connected to an end of the button and including a support rod, movable in a first direction within a through hole formed in a base plate, and a hooking projection which is locked to or released from a hooking groove; a return spring interposed between the first hooking part and the base plate so as to provide elastic force to the first hooking part; and a second hooking part which is attached to a slanted slot in a side of the first hooking part so as to move in a slantwise direction with respect to the first hooking part.