The application of infrared-visible sum-frequency generation (SFG) spectroscopy to the in situ monitoring of film growth was examined by the observation of the growth of the physisorbed layer of formic acid on a reconstructed Pt(110)-(1x2) surface. The signal by the CH stretching band displayed a characteristic interference pattern as deposition proceeded at 158 K to prove that the signal originated from inside the deposited film and the CH bond and molecular plane of formic acid was angled to the surface. The expressions for the electric fields of the SF beams generated inside the film, at the top layer, and at the bottom layer were derived by incorporating the interference effects of the exciting and generated light beams. The formula was used to simulate the observation and to estimate the growth rate.