an etalon formed by bonding a quartz crystal bulk having a pair of Z-cut surfaces and a LiCAF bulk having a pair of Z-cut surfaces with one of the Z-cut surfaces of the quartz crystal bulk facing one of the Z-cut surfaces of the LiCAF, reflection films being formed on the other of the Z-cut surfaces of the quartz crystal bulk and the other of the Z-cut surfaces of the LiCAF, respectively, and a ratio of an optical length of the quartz crystal bulk to that of the LiCAF bulk being set between 2:3 and 1:7, wherein the etalon functions as a Fabry-Perot resonator;

a photo-detector to receive the laser beam transmitted through the etalon and convert the laser beam into an electrical signal;

an error signal generator to generate an error signal from the electrical signal output from the photo-detector, the error signal used for controlling the optical frequency of the laser diode;

an oscillator to generate a dither signal;

the quartz crystal bulk including electrodes formed on a pair of X-cut surfaces, respectively, and supplied with the dither signal there between;

the error signal generator including a synchronous detector to detect synchronously the electrical signal output from the photo-detector, based on the dither signal to obtain the error signal, and the error signal used for controlling the optical frequency of the laser diode to stabilize the optical frequency to an extreme value of an optical transmittance of the etalon; and

a base comprising

a pair of shafts having first ends respectively attached to central parts of surfaces of electrodes formed on the pair of X-cut surfaces of the Quartz crystal bulk, and

a holding device configured to hold second ends of the pair of shafts, separate a bottom part of the etalon from the base and transmit the laser beam near by a central part of the etalon,

wherein the LiCAF bulk is lower in height than the quartz crystal bulk.