A lockup clutch engaging force control device of a torque converter for a vehicle which transmits torque via fluid between a pump impeller connected to an engine and a turbine runner connected to a transmission, the torque converter comprising a lockup clutch which engages the pump impeller and the turbine runner according to an engaging force, the device comprising: an engaging force regulating mechanism (3, 4) which regulates the engaging force; and a programmable controller programmed to: execute open loop control of the mechanism (3, 4) to increase the engaging force until a slip rotation speed of the pump impeller and the turbine runner reaches a predetermined value, calculate an engine output torque of the engine in the open loop control of the mechanism (3, 4); calculate a transmittable torque (TLU_z) of the lockup clutch according to the engaging force, and determine that the engine output torque has decreased when a value obtained by subtracting a transmittable torque (TLU_z) of the lockup clutch from the engine output torque falls below a first predetermined value; and execute open loop control of the mechanism (3, 4) to decrease the engaging force when the engine output torque has decreased in the open loop control of the mechanism (3, 4), in accordance with a decrease amount (|ΔTQE1|) of the engine output torque (S59-S61, S65). an engaging force regulating mechanism (3, 4) which regulates the engaging force; and a programmable controller programmed to: execute open loop control of the mechanism (3, 4) to increase the engaging force until a slip rotation speed of the pump impeller and the turbine runner reaches a predetermined value, calculate an engine output torque of the engine in the open loop control of the mechanism (3, 4); calculate a transmittable torque (TLU_z) of the lockup clutch according to the engaging force, and determine that the engine output torque has decreased when a value obtained by subtracting a transmittable torque (TLU_z) of the lockup clutch from the engine output torque falls below a first predetermined value; and execute open loop control of the mechanism (3, 4) to decrease the engaging force when the engine output torque has decreased in the open loop control of the mechanism (3, 4), in accordance with a decrease amount (|ΔTQE1|) of the engine output torque (S59-S61, S65). execute open loop control of the mechanism (3, 4) to increase the engaging force until a slip rotation speed of the pump impeller and the turbine runner reaches a predetermined value, calculate an engine output torque of the engine in the open loop control of the mechanism (3, 4); calculate a transmittable torque (TLU_z) of the lockup clutch according to the engaging force, and determine that the engine output torque has decreased when a value obtained by subtracting a transmittable torque (TLU_z) of the lockup clutch from the engine output torque falls below a first predetermined value; and execute open loop control of the mechanism (3, 4) to decrease the engaging force when the engine output torque has decreased in the open loop control of the mechanism (3, 4), in accordance with a decrease amount (|ΔTQE1|) of the engine output torque (S59-S61, S65).