In a force sensor abnormality detection system for a legged mobile robot having a force sensor installed between a foot and each leg and producing an output indicative of floor reaction force acting from a floor on which the foot contacts, the robot is controlled to to perform a walking-in-place motion when the robot is powered on, it is discriminated whether the output of the force sensor during the walk-in-place motion are within a predetermined range, and abnormality of the sensor is detected based on a result of the discrimination, thereby enabling detection of force sensor abnormality with high accuracy in a legged mobile robot whose feet are equipped with force sensors for detecting floor reaction forces.