A first light feedback element is arranged at an optical distance L1 from a front facet of a semiconductor laser from which an output light is emitted on an optical path of the output light. An i-th light feedback element is arranged at an optical distance Li from the front facet on the optical path of the output light, where i=2 to n, n is a positive integer not less than 2, and Li>L1. L1 and Li satisfies ((M−1)+0.01)