A keycap detachably provided on a guide mechanism of a key switch includes a cap main body having a front face pressed at the time of key input and a rear face on the opposite side to the front face; and a first engaging projection and a second engaging projection, both of which protrude from the rear face of the cap main body with a gap therebetween so that an engaging portion of the guide mechanism is detachably engaged into the gap. At least a part of an area around the first engaging projection and the second engaging projection in the cap main body has a thin part thinner than other parts of the cap main body. As a result, the falling off of the keycap can be prevented.