When a user logs in to an MFP and inserts a medium, a serial number of the medium is stored in a device information database in correspondence with user information of the user, and a descramble key, e.g., a hash code of the serial number is registered in the user information. When the medium is removed from the MFP which is still logged in, the registration is deleted. When the MFP into which the medium is still inserted is logged out, no registration is deleted. When the user logs in to the MFP into which the medium is already inserted, the MFP determines whether a descramble key is registered in user information of the user. When a second user logs in to an MFP from which a first user had logged out while forgetting to remove a medium, the MFP prohibits the second user from accessing the medium because no descramble key is registered in user information of the second user.