PURPOSE: To perform optimum running according to a kind of fuel, by providing a lock-up characteristics varying means which receives a signal from a fuel detecting means and varies lock-up characteristics used for a lock-up control means according to a kind of fuel.

CONSTITUTION: In the case of regular gasoline having a low octane number, since knocking occurs to a specified running region, a known knocking sensor is utilized, and by detection of the presence of knocking in a specified region, the magnitude of an octane number is detected. Operation and inoperation of lock-up clutch 15 is performed based on lock-up characteristics, and kinds of the lock-up characteristics depending upon a kind of fuel are previously stored in a memory means. A lock-up control means and a lock-up characteristics varying means are actuated by the use of a microcomputer. In case of a high octane gasoline being used, variation to lock-up characteristics in which a lock-up region is decreased is effected, and in case of regular gasoline being used, variation to lock-up characteristics in which a lock-up region is increased is performed.