A virus detection system uses a USB relay device that can support not only the use of USB mass storage but also the use of other USB devices such as a mouse. The USB relay device has a connection switching unit for switching a connection state between a first connector unit to which a USB client is connected and a second connector unit connected to a USB host controller. The virus detection system includes a virus check engine unit that performs a virus check of a file acquired from the USB client or the USB host controller via the USB relay device. On the basis of a result of the check by the virus check engine unit, the USB relay device performs switching control of the connection state by the connection switching unit.