An electric motor (10) is constructed with an inner circumference side rotor (11) and an outer circumference side rotor (12) which are coaxially arranged; and planetary gear mechanism which rotates at least one of the inner circumference side rotor (11) and the outer circumference side rotor (12) around an rotary shaft O. Long sides of substantially plate-like inner circumference side permanent magnets (11a) of the inner circumference side rotor (11) and those of substantially plate-like outer circumference side permanent magnets (12a) of the outer circumference side rotor (12) are arranged so as to face each other by the rotation of at least one of the inner circumference side rotor (11) and the outer circumference side rotor (12) in a cross section perpendicular to the rotary shaft O with the planetary gear mechanism.