For highly accurate optical frequency measurement in 1.5 micrometers wavelength region, an optical frequency comb (OFC) generator was realized by using a high frequency LiNbO3 electro-optic phase modulator which was installed in a Fabry-Perot cavity. By using the OFC generator, we demonstrated the frequency difference measurement up to 0.5 [THz] with a signal-to-noise ratio higher than 61 [dB], and the heterodyne optical phase locking with a heterodyne frequency of 0.5 [THz] in which the residual phase error variance was less than 0.01 [radian2]. The maximum measurable frequency difference, which was defined as a sideband frequency with the signal-to-noise ratio of 0 [dB], was estimated to be 4 [THz].