It is known that the low-frequency fluctuation about 1/20 of vortex shedding frequency exists in the irregularity of the vortex shedding from a two-dimensional circular cylinder. Local vortex shedding frequency in spanwise direction is considered to differ from one point to another due to different local Strouhal number caused by flow three-dimensionality. Therefore, nonlinear interaction between vortex shedding cells of different frequencies is considered to be the cause of the low-frequency fluctuation. In this study, two distinctive different vortex shedding frequencies have been forced to occur at a local spanwise position with use of a stepped circular cylinder, and modified particular phase difference TSC (Trans-Spectral Coherence) analysis was conducted to investigate the presence of quadratic (or second order) nonlinear interaction in the wake. It is expected that the quadratic nonlinear interaction is generated as the result of complicated interaction between spanwise vortex cells of different frequencies.