In the optical tomographic imaging apparatus according to an aspect of the present invention, since a stable light intensity can be secured for a reflected light beam of a measurement light beam reflected off of a transmitting surface of a probe outer casing as a returning light beam to an optical fiber, the optical path length difference between a reference light beam and the measurement light beam can be precisely adjusted by adjusting the optical path length of the reference light beam with an optical path length adjusting device. In addition, a signal of a measurement object can be reliably detected from a detection signal of an interference light beam detected by an interference light detecting device.