A planar waveguide laser device forms a waveguide by a plate-like laser medium having birefringence and clad attached to at least one of the surfaces of the laser medium perpendicular to its thickness direction, amplifies laser light by a gain produced by excitation light incident on the laser medium, and performs laser oscillation. The laser medium is formed of a material having an optic axis on a cross section perpendicular to the light axis, which is the laser travelling direction. The clad is formed of a material having a refractive index in a range between refractive indexes of two polarized lights that travel along the light axis in the laser medium and have oscillation surfaces that are orthogonal to each other. The planar waveguide laser device readily oscillates linearly polarized laser light.