inside of the discharge tubes 22 it diverges toward the outside of the resonance space due to diffraction this scattered beam 8 a diverging toward the outside is intercepted by the spacer 21 and is not delivered outside thereby preventing the scattered beam 8 a from mixing into the output laser beam 8 fig2 a b and c are diagrams showing the effects of the inside diameter and length of the discharge tubes 22 and discharge tubes 23 and inside diameter of the spacer 21 on the converging performance of laser beam in these diagrams as the parameters for evaluating the converging performance of laser beam the width of the heat affected zone by cutting a mild steel plate by laser beam was used the width of the heat affected zone was measured on the basis of the cut off line fig2 a shows the relation between r 1 r 2 change and heat affected width supposing l 2 l 1 l 2 to be 0 5 and r 3 r 2 to be 1 as clear from this diagram when the value of r 1 r 2 is larger than 1 that is when the inside diameter r 1 of the discharge tubes 22 disposed at both ends in the optical axis direction is larger than the inside diameter r 2 of the discharge tubes 23 disposed.