When the connector of an electronic card device is mated with the connector of a host device with the main power supply of the host device being turned off, a power supply control unit in the host device starts the supply of power to the card device. When no operating command is received from the host device within a fixed period after the start of reception of power, a central processing unit of the card device determines that the main power supply of the host device is turned off and starts radio communication using a radio communication unit on the basis of operating commands stored in an information storage unit.