PURPOSE: To accurately measure displacement at all times without letting an output signal disappear even if a speckle pattern is moved by forming the speckle patter quite different in pattern with the wave length of laser beam changed when the component of desired space frequency is less than a specified value in strength.

CONSTITUTION: A power judging set 131 inputs 130-1, and 130-2 two signals which are different in phase by 90°, judges whether or not the sum of squared signal level is in a level that displacement can accurately be detected based on a specified threshold, and inputs the aforesaid signals into a laser control set 133. In the control set 133, when a signal level is less than a specified value, the applied electric power of a semi-conductor laser 134 is changed, the profile of a speckle pattern formed over a light receiving surface is changed by irradiating laser beam changed in wave length, so that signals equal to or more than a specified value in strength are thereby inputted. Meanwhile, in a phase detector 132, the phase of the speckle pattern received is obtained, and the direction of its displacement is detected based on the reverse sine of the two signals. Beside, the difference in phase from phase previously obtained is accumulated, so that the quantity of displacement of a measured object is thereby determined.