1. A reciprocating engine type electric generator comprising: a reciprocating engine including at least one cylinder block having a cylinder bore, and a piston disposed in the cylinder bore of the cylinder block and reciprocating between a top dead center and a bottom dead center; an electric generator including at least one stator coil and a rotor connected to the reciprocating engine and mounted with at least one permanent magnet rotated following the reciprocation of the piston, wherein the electric generator generates an alternate voltage at the stator coil following the rotation of the rotor; means for recognizing a phase difference between a phase of the alternate voltage wave generated at the stator coil following the rotation of the rotor and a phase of a reference alternate voltage; means for controlling a speed of a rotational magnetic field generated at the stator coil by controlling the alternate voltage applied to the stator coil based upon the phase difference recognized by the means for recognizing the phase difference.