Existence of the plug potential (PP) bounce ion is quite essential for effective improvement of axial confinement in the tandem mirror, which is bounced by the confining potential hill. We paid attention to the neutral particles changed from the bounce ions through the charge exchange process and measured simultaneously both energy and emergence angle of the neutral particles by use of a charge exchange neutral particle analyzer for measuring bounce ions located near the inner mirror throat (IMT) of the plug barrier cell. We detected successfully the bounce ions during confining potential formation and assigned the bounce ions to the PP bounce ion and the OMT bounce ion which is bounced near the outer mirror throat (OMT) of the plug/barrier cell. The trajectories of the PP bounce ion were calculated, and it was found that the confinement of the PP bounce ion was sensitive to the radial profile of the confining potential in relation to the radial transport.