PURPOSE: To check oil temperature rise in a hydrostatic continuously variable transmission(HST) without lowering the relief pressure by operating an alarm at a time when a difference between setting revolution to be found out of shift operated variable information and actual outputting revolution exceeds the setting range.

CONSTITUTION: A feedback control mans F so a controller 12 operates motor- driven cylinder 8 on the basis of each detection information of a rotational frequency detector 15 of an engine 3 and a first potentiometer 10 detecting the operating position of a shift lever 9 so as to make the setting revolution of an output shaft 4a to be set up by operation of the shift lever 9 and the actual outputting revolution of the output shaft 4a accord with each other. In addition, when a difference between the setting revolution of the output shaft 4a to be found out of shift operated variable information detected by a second potentio meter 21 detecting the operating position of a trunnion shaft 4t and the actual outputting revolution of the output shaft 4a has exceeded the setting range, an alarm 22 operates via a comparing control means H.