A shift control system controls an automatic transmission of a vehicle wherein half-lock-up control for a lock-up clutch is executed when the running state of the vehicle is in a predetermined operative range. The shift control system includes a half-lock-up control inhibiting unit for inhibiting the half-lock-up control from being executed when the running state of the vehicle is held in a specific operative state even though the running state of the vehicle is in the predetermined range, a determining unit for determining whether or not the half-lock-up control is inhibited by the half-lock-up control inhibiting means, and a controlling unit for controlling the automatic transmission such that a low speed stage at which the half-lock-up control is easily executed is positively utilized when it is determined that the half-lock-up control is not inhibited by the determining unit, and moreover, controlling shift change of the automatic transmission such that a high speed stage having a gear ratio lower than that at the lower speed stage is positively utilized when it is determined that the half-lock-up control is inhibited by the determining unit.