A laser oscillator is obtained in which the flatness of an optical element that makes contact with a cooling flange is maintained in a highly precise state, by suppressing a flange presser and a base from profile-deforming the cooling flange. The flange presser and the cooling flange make contact with each other by the intermediary of three protrusions provided in either one of the flange presser and the cooling flange, and the three protrusions are arranged at the vertexes of a triangle; the cooling flange and the base make contact with each other by the intermediary of three protrusions provided in either one of the cooling flange and the base; the three points at which the flange presser and the cooling flange make contact with each other by the intermediary of the protrusions and the three points at which the cooling flange and the base make contact with each other by the intermediary of the protrusions are arranged in such a way as to face respective corresponding points; and the flange presser and the base that flank the cooling flange are fixed fastened with a fastening member.