An automatic transmission includes a gear transmission mechanism including first and second friction mechanisms which are alternatively engaged by alternative supply of first and second pressures thereto when the highest speed stage and the next to highest speed stage are alternatively engaged, and a torque converter including a lockup clutch. A transmission control system includes a pressure control system which controls the gear transmission mechanism by selectively supplying those pressures and possibly others thereto, a lockup clutch control valve which according to its switched position controls the engagement of the lockup clutch and which is switched to its engage position by a control pressure, and a lockup clutch interrupt valve, which selectively transmits the first pressure to the lockup clutch control valve as the control pressure, which is supplied with the second pressure which when present switches it positively to its non communicating condition, and which is also supplied with governor pressure which in the absence of the second pressure switches it to the communicating condition when this governor pressure is higher than a certain value, and otherwise leaves it in the non communicating condition.