A relay controller, comprising: a relay switch in which a contact point is adapted to connect a power supply to a load, and a coil adapted to be connected to the power supply; a first switching element which is connected in series to the coil; a regenerative current circuit which is connected in parallel to the coil and includes a second switching element and a diode which is connected in series to the second switching element; a first switching element control unit which is adapted to turn on the relay switch by PWM control of the first switching element and to turn off the relay switch by stopping the PWM control of the first switching element; and a second switching element control unit which is adapted to turn on the second switching element when the first switching element is PWM-controlled and to turn off the second switching element when the PWM control of the first switching element is stopped; characterized by the relay controller further comprising a current detector which is adapted to detect a value of the regenerative current flowing through the second switching element, the relay controller being able to monitor the on/off state of the relay switch by monitoring the voltage between the contact point and the load, the relay controller being thus able to detect the value of the current just before the relay switch is turned off, and to set this value as a minimum value of the current required to keep the relay switch turned on, wherein the first switching element control unit is adapted to adjust the duty cycle of the PWM control with respect to the first switching element so that a minimum value detected by the current detector matches the minimum value of the current required to keep the relay switch turned on. a relay switch in which a contact point is adapted to connect a power supply to a load, and a coil adapted to be connected to the power supply; a first switching element which is connected in series to the coil; a regenerative current circuit which is connected in parallel to the coil and includes a second switching element and a diode which is connected in series to the second switching element; a first switching element control unit which is adapted to turn on the relay switch by PWM control of the first switching element and to turn off the relay switch by stopping the PWM control of the first switching element; and a second switching element control unit which is adapted to turn on the second switching element when the first switching element is PWM-controlled and to turn off the second switching element when the PWM control of the first switching element is stopped; characterized by the relay controller further comprising a current detector which is adapted to detect a value of the regenerative current flowing through the second switching element, the relay controller being able to monitor the on/off state of the relay switch by monitoring the voltage between the contact point and the load, the relay controller being thus able to detect the value of the current just before the relay switch is turned off, and to set this value as a minimum value of the current required to keep the relay switch turned on, wherein the first switching element control unit is adapted to adjust the duty cycle of the PWM control with respect to the first switching element so that a minimum value detected by the current detector matches the minimum value of the current required to keep the relay switch turned on.