PROBLEM TO BE SOLVED: To prevent a machine body to start rotation at the site by a wrong operation when a sub-transmission is in the neutral position, in an apparatus having the sub-transmission arranged between an HST-type infinitely variable transmission structure for traveling and a differential structure for right and left wheels or crawler. SOLUTION: This operation structure of an HST-type transmission device having an HST-type infinitely variable structure 25 for traveling, a sub transmission device, an HST-type infinitely variable structure 28 for rotation, a main transmission-operating tool 68 for operating the HST-type infinitely variable structure 25 for traveling, a sub-transmission-operating device FL for operating the sub-transmission device and a steer-operating tool 19 for operating the HST-type infinitely variable structure 28 for rotation, has a lock structure for preventing the rotation of the HST-type infinitely variable structure 28 for rotation even if the steer operating tool 19 is operated when the sub- transmission device is in neutral.
