the line pressure input port 61a is discharged in a small amount via an orifice 64 from oil path l62 while acting on the spool 61c in conjunction with a cutback pressure p cb inptted from oil path l63 as the back pressure for restoring the spool 61c for slightly closing the line pressure input port 61a thus the throttle pressure p th is regulated to a hyraulic pressure corresponding to the opening degree of the line pressure input port 61a the throttle pressure p th thus regulated by the primary throttle valve 61 acts on an accumulator control valve not shown from primary regulating valve 51 39 secondary regulating valve 54 and oil path l64 for regulating the line pressure p l and the accumulator control pressure to a pressure corresponding to the throttle opening u03b8 meanwhile the cutback pressure p cb acting upwardly a 1 gt a 2 where a 1 and a 2 being active surface areas of lower and upper lands on which the cutback pressure p cb acts on downshift plug 61a via oil path l65 acts for deducting from the force of spring 61b acting on a throttle cam 63 to reduce the force of foot pressure applied on the accelerator pedal 25 an actuator 62 is mounted on the primary throttle valve 61 in such a manner that the distal end of a lever 65 the proximal end of which is rotatably supported on the valve body side abuts on the lower end of the primary downshift plug 61a at.