A laser oscillator includes a light condensing block having a through-hole for housing a laser medium, and an aperture for introducing excitation light from an excitation light source module into the through-hole; an end plate fixed to an end of the light condensing block, in which a cooling water channel for leading cooling water to the light condensing block and a cooling water channel for leading cooling water to the excitation light source module are formed; and a flow tube fixed and sealed by the end plate, for forming a cooling water passage for the laser medium; and an excitation light source module in which a cooling water channel, communicating with the water channel for the excitation light source module in the end plate, is formed for cooling its excitation light source.