PURPOSE: To ensure engine braking by causing sufficient hydraulic pressure to be supplied to an engine brake frictional clutch element even if a select position detecting switch fails and is in an abnormal state.

CONSTITUTION: An abnormality detection means 16 for a select position detecting switch 14 is provided so that when the switch is in an abnormal state a control command means 17 sends to a line pressure control means 11 an order to control line pressure to maximum pressure. Thereby hydraulic pressure supplied to an engine brake frictional clutch element 13 by a backup pressure supply means 12 is adjusted to high pressure by a backup pressure regulation means 15 when the select position detecting switch is in a normal state; also, sufficiently high hydraulic pressure is supplied to the engine brake frictional clutch element 13 even if the backup pressure regulation means is not actuated because of the breakdown of the select position detecting switch. An engine brake therefore effectively works.