To detect slipping of a driving wheel in a short of time and judge whether to prohibit speed change or not in a short of time.

It has a hydraulic transmission gear connected to an engine 10, an input rotation number detection means to detect the number of revolution on the input side of the hydraulic transmission gear, an output revolution number detection means to detect the number of revolution on the output side of the above-mentioned hydraulic transmission gear and a driving wheel slipping detection means 93 to detect that a driving wheel 62 is slipping at the time of judging whether a rotation number ratio of the number of revolution on the output side against the number of revolution on the above-mentioned input side is higher than a set value or not and finding that the above-mentioned revolution number ratio is higher than the set value. In this case, as it is only required to calculate the number of revolution of the input side of the hydraulic transmission gear and the number of revolution of the output side but it is not necessary to find variation of the number of revolution of an output shaft of a transmission gear 16, it is obviated to monitor the number of revolution of the output shaft.