PURPOSE: To obtain a speed change lock device, which restricts a shift down before a predetermined safety condition is satisfied in the time of high speed running, by constituting a device locking a speed change, when a running oil pressure is lower than the threshold value of detection oil pressure, and freeing the speed change when the running oil pressure is higher than the threshold value.

CONSTITUTION: When an electric signal from a pressure switch A507 is output, that is, when a running oil pressure is not more than a predetermined value, an electromagnetic 2-position selector valve A503 cuts off a pilot air pressure from a circuit A502, extending by tension of an urging spring a rod A510 of a pneumatic cylinder A509, and the rod houses its end in a detent A542 placing in a lock condition a selector valve A52 with a speed changer operating lever. Reversely when the electric signal from pressure switch A507 is not output, that is, when the running oil pressure is not less than the predetermined value, the pilot air pressure from the circuit A502 is guided to a circuit A504 contracting the urging spring detaching the rod end of the pneumatic cylinder A509 from the detent A542, thus the selector valve A52 with the speed changer operating lever is placed in a free condition.