A coupling structure couples first and second casing parts together. The first casing part is formed with a detachment hole. The coupling structure includes one resilient engaging hook projecting from the first casing part and a coupling member disposed between the first and second casing parts and formed with first and second engaging grooves. The resilient engaging hook hooks the first engaging groove in a right direction. The coupling member has a pushing element tending toward the detachment hole. One rigid engaging hook projects from the second casing part and engaging the second engaging groove in a left direction. Manually leftwise movement of the pushing element in the left direction results in simultaneous bending of the resilient engaging hook in the left direction so as to disengage the rigid engaging hook from the second engaging groove, thereby permitting removal of the second casing part from the first casing part.