An optical signal optical path switching method comprising steps of using a thermal lens based on a distribution of refractive index produced reversibly caused by temperature increase generated in anarea of the light-absorbing layer film of thermal lens forming devices 1, 2 and 3, that has absorbed control light beams 121, 122 and 123, and in the periphery thereof, causing the converged signal light beam to exit from the thermal lens forming device with an ordinary divergence angle when the control light beams 121, 122 and 123 have not been irradiated and no thermal lens has been formed, andcausing the converged signal light beam to exit from the thermal lens forming device with a divergence angle larger than the ordinary divergence angle when the control light beams have been irradiatedand a thermal lens has been formed, and causing the signal light beam to travel straight through holes 61, 62 and 63 of mirrors provided with the holes for the signal light beam to pass through whenthe control light beams have not been irradiated and no thermal lens has been formed, and changing the optical path by reflecting the signal light beam using the hole-provided mirror when the controllight beams have been irradiated and a thermal lens has been formed.