A computer system includes a main device and a subordinate device. Data transmission and reception by means of an optical transmission system are performed by connecting the main device and the subordinate device by an optical transmission cable. The subordinate device, a power of which needs to be cut-off prior to the main device, includes a light reception determination part and a power supply control part. The light reception determination part determines whether the main device is in a power-off state by presence or absence of light reception based on a light-receiving amount received by a photoelectric converter. When it is determined that the main device is in the power-off state, the power supply control part controls a power supply device so as to cut-off the power of the subordinate device.