The optical displacement-detecting mechanism has: a light source for irradiating a target for measurement with light; a light source-driving circuit for driving the light source; an optical detector made from a semiconductor for receiving light after the irradiation of the target for measurement by the light source and converting the light into an electric signal thereby to detect an intensity of light; and an amplifier including a current-voltage conversion circuit for performing current-to-voltage conversion on a detection signal of the optical detector with a predetermined amplification factor. In the optical displacement-detecting mechanism, a light source having a spectrum half width of 10 nm or larger is used, whereby the light source can be driven with an output power of 2 mW or larger without generating mode hop noise and optical feedback noise.