Abstract: To a laser medium comprising vapor of metal atoms A is added vapor of metal atoms B such that the difference .DELTA.E(B) between the energy value E.sub.1 (B) at the upper laser level and the energy value at the lower laser level E.sub.2 (B) as the metastable state after laser transition or fluorescent transition is substantially equal to the energy level E.sub.2 (A) of the metal atoms A at the lower laser level. Secondary collision of the metal atoms A in the E.sub.2 (A) state and the metal atoms B in the E.sub.2 (B) state with one another causes energy transition, whereby the metal atoms A do a work of exciting the metal atoms B to a resonant excitation level to lose its own energy and undergo transition to a ground level E.sub.0 (A). Consequently, the excitation lifetime at the lower laser level in the laser transition is reduced to extend the inverse population time and also the laser output pulse width so as to increase the laser conversion efficiency.