To provide a constitution where a clutch piston of an auxiliary transmission for enabling the start control under the increase of torque, does not receive the returning force from lubricant pressure inside of the auxiliary transmission.

In starting, the auxiliary transmission 20 is kept in a low-speed stage (decelerating stage) selecting state by releasing a clutch 24, and the start is performed by controlling the connection of a forward clutch 4a from a neutral state where both of the forward clutch 4a and a backward brake 4b are released. As a piston 24p of the clutch 24 is pressed to the right in a drawing by the lubricant pressure in an auxiliary transmission case 21, the piston working pressure α to the piston chamber 46 should be determined to be higher for the same, which worsens the fuel consumption in a conventional mechanism, but by mounting a back chamber 53 in a state of being kept into contact with the piston 24p at a side farther from the piston chamber 46, the piston pressing force by lubricant pressure in the auxiliary transmission case 21 is reduced, and further as the piston working pressure α is not determined to be higher, the worsening of the fuel consumption can be prevented.