An electronic apparatus system of this invention includes an electronic apparatus, and a fuel cell unit which is detachable from the electronic apparatus. The fuel cell unit incorporates a DMFC that can produce electricity by chemical reaction. On the other hand, the electronic apparatus has at least two operation modes having different power consumption amounts such as a normal mode and power saving mode. Upon switching the operation mode, the electronic apparatus advises the fuel cell unit accordingly. Upon reception of this advice, the fuel cell unit compares the current output electric power of the DMFC with the power consumption amount in the operation mode after switching, and returns a message based on that comparison result. The electronic apparatus switches the operation mode based on the returned message.