A plane waveguide type laser includes: a plate-shaped laser medium; a semiconductor laser which causes excitation light to enter an end surface of the laser medium; first and second claddings which are bonded to lower and upper surfaces of the laser medium, respectively, to form a waveguide in a vertical direction; a comb heat sink bonded to a lower surface of the first cladding; and a thermal lens producing portion bonded to an upper surface of the second cladding. In this structure, laser oscillation in the vertical direction is performed in a waveguide mode of the laser medium, and the thermal lens producing portion forms a periodic lens effect in the laser medium to perform laser oscillation in a lateral direction in a plurality of resonant modes.