Mass spectra of high-Rydberg (HR) species produced by 60 eV electron impact on methane have been observed. The relative intensity among HR CH4, CH2 and CH was in good agreement with that among the corresponding ions obtained by a normal operation of the mass spectrometer. In contrast, the relative intensity of HR CH3 to HR CH4 was a half of that of CH3+ to CH4+. This finding is due to the fact that a considerable amount of CH3+ ions are produced through autoionization from the 3pt(2) and 4pt(2) Rydberg states, whereas HR CH3 cannot be produced through these Rydberg states.