Ba⁺ and Yb⁺ ions were trapped with buffer gas in a rf ion trap. The number and the storagetime of the trapped ions were determined by the rf resonance method. It was shown thatthe motion of the trapped ions was expressed by an external forced oscillation equation with anonlinear term in the case the fabricated trap was imperfect. Due to this nonlinear effect, thesubharmonic oscillation was induced, and thus the number and the storage time of the trappedions were decreased and shortened, respectively. The temperature and the spatial distributionof the trapped ions were determined by the fluorescence detection method.