A power tool dust collecting device includes, in a housing in which a suction opening is protrudingly provided and on which a power tool is mountable, a motor chamber holding a motor having a brush that comes into sliding contact with a commutator of a rotor rotating integrally with an output shaft, a fan chamber housing a dust collecting fan attached to the output shaft of the motor, and a dust collecting chamber in which a filter, through which passes air sucked up from the suction opening due to rotation of the dust collecting fan in accordance with driving of the motor and which collects dust in the air, is mounted on an upstream side of the fan chamber. A cushioning member is interposed between an inner wall of the motor chamber and an outer peripheral surface of the motor.