To extend the temperature range for a stable operation without mode hopping and obtain a stable laser oscillation, a laser oscillator is provided. The laser oscillator includes: an optical circulator in which light incident on a first port is exited from the second port, light incident on the second port is exited from a third port, and light incident on a fourth port is exited from the first port; a first optical amplification fiber that amplifies the light exited from the third port because of being excited by an exciting light and injects the same into the first port; a reflective optical filter that reflects light with a predetermined wavelength among the light exited from the second port and injects the same into the second port again; and an pump light source that generates an exciting light to excite the first optical amplification fiber. The reflective optical filter transmits light oscillated by laser in a ring resonator including the first optical amplification fiber, the optical circulator and the reflective optical filter and outputs the same. The pump light source injects the exciting light from the fourth port into a ring resonator.