To provide a fiber type optical switch avoiding the difficulty of optical axis alignment and high cost caused by a mechanism that light is once taken out to an external part of a fiber to perform ON/OFF switching since the optical fiber can not be formed by using the fiber and the light is reflected again to the fiber, in a fiber laser device wherein an activated ion is doped in the fiber and inversion distribution is made to appear by utilizing ion level excitation.

An LD for excitation is connected to an Er-doped optical fiber via a WDM to excite an Er ion by using the LD for excitation. The Er fiber is made transparent to oscillation light λo by exciting the Er ion by using the LD for excitation to fill the Er ion with a first excitation level. This means ON switching. When the Er ion is in a ground state, the switch is in an OFF state since the oscillation light λo is absorbed.