A signal modulation unit modulates an actuator control signal for controlling an actuator by using a modulation signal. A correlation calculation unit calculates, when a measurement signal indicating a physical state measured by a passive sensor is received from the passive sensor after a modulated actuator control signal being the actuator control signal having been modulated by the signal modulation unit is transmitted to the actuator, a correlation between the measurement signal received and a modulation signal used for modulation of the actuator control signal by the signal modulation unit. The signal determination unit determines whether or not the measurement signal is a legitimate signal by using a correlation value obtained by the correlation calculation unit.