An optical measuring device is provided. An actuator of a reference mirror set drives a reference mirror to move back and forth at a scan velocity. A first light source module transmits a first light beam to an optical coupling module transmitting two parts of the first light beam respectively to an examinee object and the reference mirror set. The first light beam then is reflected by the examinee object and reference mirror set and then transmitted to the optical coupling module and the processing unit. The second light source module transmits a second light beam to the examinee object. Then the second light beam is reflected and then transmitted to the second sensing unit. The second sensing unit provides a sensing signal to the processing unit which accordingly provides a value of the relative velocity. The thickness is calculated according to the relative velocity and the scan velocity.