A hydraulic control unit (18), for a power transmission system having an oil receiving device (26,37), which controls a power transmitting condition of the power transmission system, and an oil reserving device for feeding the oil to the oil receiving device, wherein the oil reserving device comprises a piston (59) in which a diametrically large portion and a diametrically small portion are arranged integrally and coaxially, a first hydraulic chamber (57) in which the diametrically large portion, and a second hydraulic chamber (58) in which the diametrically small portion are housed liquid-tightly and movably back and forth; and an oil feeding amount control device for feeding the oil of the first hydraulic chamber to the oil receiving device, by raising the oil pressure in the second hydraulic chamber (58) to operate the piston (59).