A disk braking device suppresses warp of friction members and deterioration in reaction at the time of braking, and provides friction members with a uniform service life, where pistons are increased in number to heighten a braking force. A piston mechanism comprises pistons arranged along an outer periphery of a disk plate. The pistons are allotted to a plurality of piston portions that have different cylinder areas s1 and (s2+s3). A pad unit is divided into a plurality of pad unit portions that have different total pressure contact areas p1 and (p2+p3) corresponding to the cylinder areas s1 and (s2+s3).