The present invention provides an optical amplifying device which can be easily downsized, increased in output, and stabilized. An optical amplifying device 1A includes an optical amplifier 10A and an energy supplier 30. The optical amplifier 10A includes an optical amplifying medium 11 and a transparent medium 12. The energy supplier 30 supplies excitation energy (for example, excitation light) to the optical amplifying medium 11. The optical amplifying medium 11 is supplied with the excitation light to amplify light and output it. To-be-amplified light passes through the transparent medium 12 in the optical amplifying medium 11 a plurality of times. The transparent medium 12 can propagate the to-be-amplified light, for example, zigzag inside.