Optical measurement of sound play an important role to understand the physical natures and spatial distributions of sound. The optical measurement of sound is especially useful under the conditions where microphones cannot be installed such as inside high-temperature fields, narrow spaces, and flow fields. Flow-induced sound generated by parallel plates is one of such situations. The measurement of the sound by microphones is difficult because the spaces between plates are narrow and the objective sound is inside flow fields. While previous research investigates the pressure field including sound fields between flat plates by computer simulations, experimental measurement have not been well investigated. In this paper, we performed the measurement of the aerodynamic sound generated by flat plates inside a flow field by parallel phase-shifting interferometry in order to experimentally capture the sound field including spaces between flat plates. The result shows that the phases of sound between plates were opposite, which is consistent with the simulation in prior research.