This unit is equipped with a solenoid valve which is installed in an oil passage and exhausts the oil pressure from the oil passage at an appointed time in accordance with the output of an electric control circuit, and with a shift transition control valve controlled by said solenoid having two independent oil chambers in which the oil pressures supplied to hydraulic servos in forward and reverse drives respectively are regulated. Shift valves are installed between the shift transition control valve and the hydraulic servos, and the oil pressure adjusted by the shift transition control valve is supplied to the appointed oil pressure servos through each shift valve from the start to the completion of a shift. When the shift is completed, the oil pressure supplied to the hydraulic servo is changed to line pressure. The timing function required for a clutch-to-clutch shift is controlled by a timing device composed of a pressure hold valve and a flow control valve.