by fluid sealed in a space between the pump impeller 2 a and the turbine runner 2 b in this state the pump impeller 2 a and the turbine runner 2 b can slip with respect to each other the force for locking up the lock up clutch 2 c is determined by the differential pressure of the applied pressure p a and the release pressure p r if the applied pressure p a is lower than the release pressure p r the lock up clutch 2 c is in the unlock state if the applied pressure p a is greater than the release pressure p r and the resultant differential pressure is greater than a predetermined value the lock up clutch 2 is in the lock up state the applied pressure p a and the release pressure p r are controlled by a control system described below a lock up control valve 11 provides the applied pressure p a and the release pressure p r on the basis of the signal pressure p s from a lock up solenoid 13 the lock up solenoid 13 generates the signal pressure p s according to a signal d output from a controller 12 this signal changing over between on and off the construction of the lock up control valve 11 and the lock up solenoid 13 is shown in fig2 which is known in the art the lock up solenoid 13 uses a fixed pilot pressure p p as a base pressure and generates the signal pressure p s according to the signal d from the controller 12 the lock up control valve 11 comprises a spool which receives the release pressure p r and the above signal pressure p s in one direction and the applied pressure p.