A laser oscillator includes a ring resonator. The ring resonator includes an optical circulator having first, second, third, and fourth ports and a first optical amplification fiber connected to the optical circulator. Light incident on the first port is exited from the second port, and light incident on the second port is exited from the third port. The fourth port provides an exciting light and injects the exciting light into the ring resonator through the first port. The first optical amplification fiber amplifies light exited from the third port with the exciting light provided by the fourth port. The laser oscillator also includes an optical member connected to the optical circulator. The optical member reflects at least a part of the light exited from the second port and injects the same into the second port again.