A laser device and an internal combustion engine. The laser device includes a light source configured to emit light, an optical system configured to concentrate the light emitted from the light source, a housing configured to accommodate the optical system, and a window disposed to the housing, to which the light passed through the optical system is incident. The window includes an optical window through which the light exited from the optical system passes, an optical window holding member configured to hold the optical window, a joint configured to join the optical window to the optical window holding member, and a protective layer disposed to a face of the joint. The internal combustion engine includes the laser device according to claim, and a combustion chamber configured to burn fuel to produce inflammable gas. In the internal combustion engine, the fuel is ignited by the laser device.