A solid state laser apparatus in which a fundamental laser beam generated in a laser medium is resonated so as to pass a nonlinear optical crystal element provided within a resonator to thereby generate a secondary harmonic laser light and a first optical element for suppressing a coupling caused by a sum-frequency generation between two polarization modes of the fundamental wave laser beam is provided within the resonator includes of a control device for controlling an effective resonator length of the resonator such that the two polarization modes of the fundamental wave laser beam become the same in oscillation intensity. Therefore, a mode hopping noise caused by the mode coupling of the fundamental laser beam within the same polarization mode can be avoided and the oscillation can be stabilized.