fully open the second pressure control servo valve 50 thereby rendering the second line pressure pl2 equal to the first line pressure pl1 in this condition the fluid in the second pressure line 46 may flow into the input side hydraulic cylinder 34 because the second line pressure pl2 now equal to the first line pressure pl1 is higher than the pressure 34 pc 34 in the input side cylinder 34 if the first line pressure pl1 is found in step ss5 to be lower than or equal to the pressure 34 pc 34 step ss7 is executed to set the second pressure control value 34 v2 34 to a comparatively small value 34 u03b1 34 the minimum value required to increase the speed ratio 34 e 34 at a relatively low rate when the speed ratio 34 e 34 is increased at a relatively low rate the second pressure control servo valve 50 is controlled so that the second line pressure pl2 is equal to or slightly higher than the first line pressure pl1 thus minimizing the power loss in the drive for the oil pump 44 when steps ss2 and ss3 reveal that the increase rate 34 e 34 is smaller than 34 b 34 but larger than 34 a 34 the controller goes to step ss8 wherein the second pressure control value 34 v2 34 is determined so that speed ratio 34 e 34 is increased at a varying medium rate this determination is based on the speed ratio 34 e 34 and the increase rate 34 e 34 according to the predetermined relation indicated in fig5 specifically the pressure differential 34 u03b4p 34 between the first and second line pressures pl1 and pl2 is increased thereby increasing the increase rate.