A plane waveguide type laser according to the present invention includes: a plate-shaped laser medium (5); a semiconductor laser (1) which causes excitation light to enter an end surface (5a) of the laser medium (5); first and second claddings (4a and 4b) which are bonded to lower and upper surfaces of the laser medium (5), respectively, to form a waveguide in a vertical direction; a comb heat sink (2) bonded to a lower surface of the first cladding (4a); and thermal lens producing means (20) bonded to an upper surface of the second cladding (4b). In this structure, laser oscillation in the vertical direction is performed in a waveguide mode of the laser medium (5), and the thermal lens producing means (20) forms a periodic lens effect in the laser medium (5) to perform laser oscillation in a lateral direction in a plurality of resonant modes. Therefore, a resonator having a small round-trip loss maybe formed without depending on a size of a thermal lens produced by excitation, and hence a rising characteristic of a pulse operation or a CW operation may be smoothed to obtain high-power stable output.