PROBLEM TO BE SOLVED: To provide a driving force distributing device having reduced size and weight and reduced cost without sacrificing the responsiveness. SOLUTION: The driving force distributing device comprises a friction clutch 16 for transmitting the power of a main driving shaft to a sub driving shaft, a piston 36 for generating pushing force on the friction clutch 16, a cylinder chamber 35 storing the piston 36 in an axially movable manner, a pressure sensor 70 for detecting hydraulic pressure in the cylinder chamber 35, a hydraulic pump 48 for supplying pressure oil to the cylinder chamber 35, a motor 42 for positively/reversely driving the hydraulic pump 48, and a controller 40 for controlling the motor 42 to be positively/reversely driven while comparing a command from a vehicle with a signal from the pressure sensor 70. A hydraulic control valve unit 44 supplies pressure oil to the cylinder chamber 35 during the positive rotation of the hydraulic pump 48, prevents the flow of the pressure oil out of the cylinder chamber 35 during the positive rotation stop of the hydraulic pump 48, and forcibly discharges the pressure oil from the cylinder chamber 35 during the reverse rotation of the hydraulic pump 48. COPYRIGHT: (C)2009,JPO&INPIT