A sheet folding device includes two rollers, a blade, two parallel pins, and two gears. The first and second parallel pins are passed through first and second pin through-holes penetrating through first and second shaft portions in a direction perpendicular to a longitudinal direction of the shaft portions. The first and second gears have first and second shaft through-holes through which the first and second shaft portions are passed, and first and second grooves in which the first and second parallel pins are inserted, respectively. Projection portions formed on inner side wall of the second groove and projecting from both sides of the second shaft portion cause the second parallel pin to closely contact a side wall that is, in the second groove, opposite to the projection portions, and cause the second parallel pin to closely contact a portion of the second pin through-hole.