An air pump comprising: a piston assembly of a pair of pistons and an armature disposed between the pair of pistons to connect the pair of pistons in a state that the pair of pistons are aligned with each other in an axial direction of the pistons; a pair of electromagnets provided at opposite sides, respectively, of the armature in a direction perpendicular to the axial direction, wherein, when an alternating electric current is applied to the electromagnets, the electromagnets generate an alternating magnetic field traversing the armature to reciprocate the armature in the axial direction of the pistons; and a pump casing comprising a pair of cylinder chambers slidably accommodating the pair of pistons, respectively, and a drive chamber allowing the armature to move between the pair of cylinder chambers and accommodating the electromagnets; characterised in that the pump casing comprises a casing body comprising side walls opposing each other in the axial direction, the side walls comprising circular cylindrical inner peripheral surfaces extending through the side walls, respectively to define the cylinder chambers, the circular cylindrical inner peripheral surfaces being slidable relative to the associated pistons, the casing body further comprising a cylindrical peripheral wall extending between the mutually opposing side walls to define the drive chamber, the casing body comprising an electromagnet-loading opening provided to extend through the cylindrical peripheral wall from the drive chamber to an outside of the cylindrical peripheral wall, wherein the piston assembly is inserted into the casing body in the axial direction through the circular cylindrical inner peripheral surface of one of the side walls so that the pair of pistons are installed in the associated circular cylindrical inner peripheral surfaces; at least one electromagnet pedestal member for positioning the pair of electromagnets at the opposite sides of the armature, the electromagnet pedestal member being inserted into the drive chamber through the electromagnet-loading opening and engaged with an inner peripheral surface of the peripheral wall, the electromagnet pedestal member engaging with the electromagnets inserted from the electromagnet-loading opening to position the electromagnets relative to the armature; head covers attached to the casing body after the piston assembly has been loaded thereinto, from left and right sides, respectively, of the casing body, the head covers defining the cylinder chambers together with the circular cylindrical inner wall surfaces. a piston assembly of a pair of pistons and an armature disposed between the pair of pistons to connect the pair of pistons in a state that the pair of pistons are aligned with each other in an axial direction of the pistons; a pair of electromagnets provided at opposite sides, respectively, of the armature in a direction perpendicular to the axial direction, wherein, when an alternating electric current is applied to the electromagnets, the electromagnets generate an alternating magnetic field traversing the armature to reciprocate the armature in the axial direction of the pistons; and a pump casing comprising a pair of cylinder chambers slidably accommodating the pair of pistons, respectively, and a drive chamber allowing the armature to move between the pair of cylinder chambers and accommodating the electromagnets; characterised in that the pump casing comprises a casing body comprising side walls opposing each other in the axial direction, the side walls comprising circular cylindrical inner peripheral surfaces extending through the side walls, respectively to define the cylinder chambers, the circular cylindrical inner peripheral surfaces being slidable relative to the associated pistons, the casing body further comprising a cylindrical peripheral wall extending between the mutually opposing side walls to define the drive chamber, the casing body comprising an electromagnet-loading opening provided to extend through the cylindrical peripheral wall from the drive chamber to an outside of the cylindrical peripheral wall, wherein the piston assembly is inserted into the casing body in the axial direction through the circular cylindrical inner peripheral surface of one of the side walls so that the pair of pistons are installed in the associated circular cylindrical inner peripheral surfaces; at least one electromagnet pedestal member for positioning the pair of electromagnets at the opposite sides of the armature, the electromagnet pedestal member being inserted into the drive chamber through the electromagnet-loading opening and engaged with an inner peripheral surface of the peripheral wall, the electromagnet pedestal member engaging with the electromagnets inserted from the electromagnet-loading opening to position the electromagnets relative to the armature; head covers attached to the casing body after the piston assembly has been loaded thereinto, from left and right sides, respectively, of the casing body, the head covers defining the cylinder chambers together with the circular cylindrical inner wall surfaces.