We have investigated effects of interaction terms on non-perturbative particle production. It is well known that a time-varying background induces abundant particle production, such as the preheating theory. As our conclusion, it is possible to induce particle production even if particles do not couple to the background directly. Such particles are produced through the interactions with other fields, which couple to a time-varying background. Especially, the resonant proccess due to the coherent oscillation of the background induces sizable amount of particles which do not couple to the background directly. In this talk, we will explain with a simple but illustrative model and show analytic and numerical results.