A hydraulic control system comprises a lock-up valve including a spool movable between a clutch engagement position and a clutch release position, and a spring biasing the spool toward the clutch release position. The spool has a pressure acting area which is exposed to a first fluid pressure which is present during operation in each of a m th speed ratio and a m+1 th speed ratio. Means is provided which is responsive to a second fluid pressure which is present during operation in one of the m th speed ratio and the m+1 th speed ratio and absent during operation in the other speed ratio. The means is operative to apply the second fluid pressure to the pressure acting area of the spool of the lock-up valve instead of the first fluid pressure.