Disclosed is apparatus and method for improving the signal by changing the voltage applied to an analyzing trap of a high resolving power Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometer. More specifically, after the ion activation, a voltage different from that of a trap electrode is applied to an additional electrode in the center of the trap electrode, and the voltage is maintained until the end of a detection cycle. Applying the above method, the stability of the ions confined in a trap is more increased, and therefore, the detected time domain signal is being lengthened. The lengthened time domain signal results in an increase of the frequency or an improvement of the resolving power and the sensitivity of the mass-to-charge domain signal.