PURPOSE: To reduce heat lens operation due to the temperature rise of a transmission part until it is ignored by providing ≥1 light transmission body, a means which insulates thermally the outer peripheral part including the peripheral flank of the light transmission body, and a means which cools one transmission surface of the transmission body.

CONSTITUTION: The light transmission body 20 is a plane circular transmission body composed of an output mirror, etc., constituting one resonator of a solid laser device or gas laser device and the outer peripheral part including the peripheral flank is held by an annular heat insulator 21 which has specific thickness. A nonmetallic material having small heat conductivity such as ethylene tetrafluororesin and ceramic is used suitably for the heat insulator. The light transmission body 20 is provided with a cooling means, which blows cooling gas 22 to one transmission surface of the light transmission body 20 at a high speed. The outer peripheral part of the light transmission body 20 is insulated thermally, so no heat flows in a radial direction (r) and the heat distribution 24 in the radial directions (r) is uniform and linear. Consequently, heat lens operation is removed from the light transmission body 20.