An air pump structured to facilitate the installation of a pair of pistons includes a casing (17) having a pair of cylinder chambers (14) slidably accommodating a pair of pistons (16), respectively, and a drive chamber accommodating electromagnets between the pair of cylinder chambers. The pump casing has mutually opposing side walls and a cylindrical peripheral wall defining the drive chamber. The side walls have circular cylindrical inner peripheral surfaces (28-1) of the cylinders defined therein, respectively. A piston assembly having the pistons connected to the opposite ends of an armature (34) is inserted into the pump casing through one of the cylindrical inner peripheral surfaces and installed therein. The peripheral wall has an electromagnet-loading opening (26-2) through which electromagnet pedestal members (26-7) and the electromagnets are inserted into the casing and secured therein.