PURPOSE: To permit the speedy speed change in case of engine brake by installing a low speed stage accumulator into a subspeed change gear and allowing the hydraulic pressure to act into the back pressure chamber of the accumulator in case of engine brake and generating the time lag in the tightening of a low speed stage frictional element.

CONSTITUTION: When the spool 102 of a shift valve 100 is switched downward by a shift solenoid 118, in order to switch a subspeed change mechanism to a low side, an oil passage 110 is drained, and a high speed stage frictional element 32 is released through a one-way orifice 122 and an accumulator 128. At the same time, a line pressure is supplied into an oil passage 112, and a low speed stage side frictional element 33 is gradually tightened through a one-way orifice 150 and a low speed stage side accumulator 132. In this case, if the third range pressure is outputted in an oil passage 190 in the high state, said pressure acts into the back pressure chamber 160 of the accumulator 132, and a piston 154 lowers, and the low speed stage side frictional element 33 is speedily engaged, and the engine brake effect can be obtained. Thus, the speed change shock can be reduced in the ordinary case, and speedy speed change is permitted in case of engine brake.