To suppress the amplification of spontaneous emission light in a principal plane width direction to thereby suppress a gain in directions other than a beam axis direction and output a high-power laser, in a solid-state laser element of a plane waveguide type that causes a fundamental wave laser beam to oscillate in a beam axis direction in a laser medium of a flat shape and forms a waveguide structure in a thickness direction as a direction perpendicular to a principal plane of the flat shape in the laser medium, inclined sections 12 are provided on both sides of the laser medium, the inclined sections 12 inclining a predetermined angle to reflect spontaneous emission light in the laser medium to a principal plane side of the flat shape, the spontaneous emission light traveling in the beam axis direction and a principal plane width direction as a direction perpendicular to the thickness direction.