PURPOSE: To improve safety by providing a doze detecting section for detecting the dozing state of driver to release the automatic constant speed travelling state while preventing erroneous switching from manual to automatic travelling states during doze.

CONSTITUTION: Upon dozing of driver under manual travelling, a doze detecting section 11 will produce a detection signal to excite a coil 12a in relay 12 thus to turn off normal-close contacts 12b, 12c. Here, the relay 12 will block the power supply circuit for a speed control section 8 comprised of positive pole of D.C. power source 1, ignition switch 2, main switch 4, brake switch 6 and clutch switch 7 from the switch signal circuit of set switch 5. Consequently, even for erroneous turning on of main switch 4 and set switch 5, the speed control section 8 will never function to prevent switching to automatic constant speed travel.