A lock-up control system for a lock-up type automatic transmission. A load detection circuit generates a reference acceleration signal that indicates a reference acceleration, and an actual acceleration signal that indicates an actual acceleration of an automotive vehicle. The circuit compares the actual acceleration signal with the reference acceleration signal for generating a first command signal when the actual acceleration signal is greater than the reference acceleration signal and generating a second command signal when the actual acceleration signal is smaller than the reference acceleration signal. The first command signal is fed to a first vehicle speed comparator means to render it operative. The second command signal is fed to a second vehicle comparator means to render same operative. Each of the first and second comparator means compares the vehicle speed signal with a set of reference vehicle speeds for generating a plurality of patterns of signals that indicate a corresponding plurality of lock-up ranges.
