PURPOSE: To prevent semi-automatic mechanical transmission system with automatic preselect operation mode from being shifted accidentally to the neutral of a transmission when the condition where vehicle speed remains constant and gear shifting does not occur continues for a prescribed time.

CONSTITUTION: The rotational speeds of the input shaft 16 and the output shaft 90 of a transmission 10 are detected by a first sensor 98 and a second sensor 100 respectively, and shifting of the transmission is monitored. If the condition where the speed remains constant for a prescribed time (e.g. 60 seconds) and gear shifting does not occur continues, the operation is returned to a manual preselection mode. At that time, a driver should preselect a desired gear ratio manually. It is thus possible to prevent accidental shifting to neutral.