A mobile electronic apparatus in which various functions can be easily added without impairing the mobility and without making any change in the hardware. The mobile electronic apparatus is carried by its authorized user to perform various types of electrical information processing, and has an attachment part detachably attached to a main body of the apparatus for performing a predetermined function, which attachment part has an input/output section for inputting/outputting information. The apparatus also has an interface section for transferring input/output signals relating to the information between the attachment part and the main body. The invention is applicable in mobile electronic apparatus having an attachment part detachably attached thereto, internet appliances such as cell phones, mobile electronic information terminals, and PDAs.