This paper describes the numerical calculations on the vibrations of a structure excited by two unbalanced rotors. When two rotors rotate on a base plate, which is free to vibrate horizontally, some kinds of vibration modes, i.e., in-phase synchronization, out-of-phase synchronization, and non-synchronization are observed. According to our investigation of rotor unbalance, we observed two types of non-synchronization. One is quasi-periodic vibration, and the other is chaos.