We determined equivalent circuit parameters of a 1.5 cell S-band RF gun cavity from the resonant characteristics of its decoupled cavities(half cell and full cell) using the code SUPERFISH. Equivalent circuit parameters of the 1.5 cell RF gun cavity resonated in the 0-mode were obtained easily from the circuit parameters of each decoupled cavities. In order to obtain equivalent circuit parameters for the $pi$ -mode cavity, we calculated the differences of the resonant frequencies and the equivalent resistances between the 0- and $pi$ -modes with slight variations of the radius and thickness of the coupling iris. From those differences, we obtained R/Q value and equivalent resistance of the $pi$ -mode, which are directly related to the equivalent circuit parameters of the coupled cavity. Using calculated R/Q value, we can express equivalent inductance, capacitance and resistances of the RF gun cavity resonated in the $pi$ -mode, which can be useful for analyzing coupled cavities in a steady state.