The fluid-structure interaction, especially the interaction between the elastic body and the ambient flow, is one of very complicated problems to be analyzed in engineering. The body is oscillated freely, consequently the information from the wake is fed back into the body motion. The interaction is a strong nonlinear one and it is so difficult to analyze it experimentally. In this article the problem on the oscillation of an elastic body is analyzed numerically, in which the features of the lock-in regime and the wake structure are made clear. The existence of the harmonic lock-in is pointed out, which is called the "pseudo-lock-in".