A DC adapter that can be housed in a battery housing portion of an electronic apparatus selectively with a primary and secondary battery. The DC adapter is connected to an AC/DC converter and is inserted into a battery housing portion of an electronic apparatus so as to supply the electronic apparatus with a DC power, wherein a substantially cylindrical adapter main body has an insertion limiting portion which is guided by an insertion guide portion of the battery housing portion when the DC adapter is inserted into the battery housing portion with the proper polarity, and comes into contact with the contact portion of the battery housing portion when the DC adapter is inserted into the battery housing portion with the reversed polarity so as to limit the insertion of the DC adapter. The insertion limiting portion is formed by projecting a part of the outer circumference of the cylindrical adapter main body in a rectangular manner along the longitudinal direction of the main body and has two sides each of which extends from a part of the outer circumference of the DC adapter. The center angle of the circular arc obtained by connecting the proximal ends of the two sides on the outer circumference of the DC adapter is less than 90.degree..