An expansion device removably attachable even to a thin portable information apparatus and an electronic apparatus including the portable information apparatus and the expansion device are disclosed. An expansion device includes: a device chassis having a shorter front-rear dimension than a front-rear dimension of an apparatus chassis of a portable information apparatus; a plurality of attracted bodies such as a plurality of magnets or iron plates provided to face a top surface of the device chassis and capable of attracting a plurality of magnets provided to face a bottom surface of the apparatus chassis; and a fitting hole that is provided at the top surface of the device chassis and into which a leg portion protruding from the bottom surface of the apparatus chassis is fittable.