In the present invention, an attack start time identification unit (223) identifies an attack start time which is the time when an attack is started on a sensor (112) which outputs sensor data at time points, such identification carried out on the basis of the sensor data at the time points which indicate the state, at the time points, of a control subject (101) on which an actuator (111) acts. An attack negation signal generation unit (224) generates, on the basis of at least one of a sensor data sequence subsequent to the attack start time and an actuator control signal sequence subsequent to the attack start time, an attack negation signal sequence which is an actuation control signal sequence for returning the state of the control subject to the state at a time prior to the attack start time.
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