A side switch mechanism which provides comfortable operation of a pair of side switches of a stylus pen and implements facilitated construction and assembly. The side switch mechanism has a side switch member having a surface construction and a rear construction. The surface construction includes a pair of knobs which are longitudinally spaced apart from each other so as to press either of two specific switching elements when the switching elements are respectively turned on. The rear surface construction includes a pair of switch pressing sections made of projections, each being provided on the rear surface so as to correspond to each knob. The switch pressing section turns on only one of the two switching elements when a corresponding one of the pair of knobs is pressed. The rear surface construction further includes fitting portions for attaching the side switch member to the casing. The side switch mechanism also has fulcrums integrally formed in the casing to support the fitting portions, so that the side switch member pivots on the fitting portions which serve as fulcrums.