The alleged in-plane polar structure in two-dimensional (2D) smectic-C liquid crystalline phases of Langmuir monolayers was directly confirmed by the optical second harmonic generation (SHG) spectroscopy for the first time. The SH signal associated with the in-plane polar structure was found to be comparable with that coming from the out-of-plane polar distribution of molecular orientation inherent at the air-water interface. This result indicates that there exits a strong preference for the heads and tails of the molecules to spontaneously associate one another over the otherwise isotropic water surface as a result of the 2D liquid crystalline order. When the monolayer underwent a first order phase transition from SmC to a solid-like phase under compression, both of the in-plane and out-of-plane components of SH signals were considerably reduced despite an increase in density, implying the rather subtle nature of the centro-symmetry breaking in the SmC phase ©2000 American Institute of Physics.