We propose a new analysis method to detect quadratic phase coupling (QPC) behavior of human gastric interdigestive pressure activity that has been acquired by a telemetric capsule-like mini-robot. The method is referred to as diagonal slice spectra. They are the Fourier transforms of the diagonal slices of the triple correlations, and can actually detect the phase coupling and coupled components respectively by expanding the real process into the complex counterpart through Hilbert transform. In order to learn more about the QPC structure in a certain frequency band that we are mostly interested in and obtain a higher frequency resolution, the method, named the wavelet packet based diagonal slice spectrum, is introduced. It shows that the nonlinear QPC behavior occurs during gastric contractions (phase II), whereas no distinct phase coupling occurs during gastric motor quiescence (phase I). It is the nonlinear cell-to-cell coupling mechanisms, existence of fast and slow waves and their interactions that nonlinear QPC structure of the gastric pressure activity occurs.