An embodiment with a dual-stage reflectron is as follows: (1) On the assumption that a reflector has a base potential X A (U) created by uniform electric fields, its design parameters are adjusted so as to cancel the first and second order derivatives at energy E=E 0 of a total time of flight T(E), and a second-order focusing position on a central axis at which the potential value becomes zero is determined (Mamyrin solution). (2) A correcting potential X C (U) to be superposed on X A (U), beginning from the second-order focusing position, is calculated so that T(E) of ions reflected in a region deeper than the second-order focusing position will be constant. (3) Voltage values of the reflector electrodes are determined so that a real potential X R (U)=X A (U)+X C (U) is created on the central axis. The superposition of the correcting potential extends the range of energy compensation up to an infinitely high order, beyond the second-order energy compensation attained by the base potential of the Mamyrin solution, thus achieving complete isochronism for the ions reflected in the correcting potential region. The real potentials before and after the starting point of the correcting potential are smoothly connected, and the discrepancy from the uniform electric field is minimized, whereby a divergence of the ion trajectory and a temporal aberration due do the off-axis location are suppressed to the minimum.