A time-of-flight mass spectrometer including an ion ejector for accelerating target ions by imparting a certain amount of energy to the ions, an ion reflector for reflecting ions ejected from the ion ejector and turning the ions around by an effect of an electric field, an ion detector for detecting the ions reflected by and exiting from the ion reflector, and a reflector driver configured to drive the ion reflector so as to create a reflecting electric field inside the ion reflector, wherein: with X denoting a coordinate along a central axis of the ion reflector, the reflector driver applies a voltage to the ion reflector so as to create, inside an inner hollow area of the ion reflector and along the central axis of the ion reflector, a predetermined potential distribution U A in which the potential monotonously changes over the entire ion reflector and therefore an inverse function X A can be uniquely obtained, thus creating an N-th order focusing position at a position with coordinate X 0 and potential E 0 inside the ion reflector, the N-th order focusing position being a position on the central axis at which a potential value is equal to the energy at which the first through N-th order derivative values of a total time of flight T of the ions with respect to an initial energy E of the ions are equal to zero; and the reflector driver also applies a voltage to the ion reflector within a space having the N-th order focusing position with coordinate X 0 as a starting point and extending into a deeper region, so as to superpose, on the predetermined inverse function X A, a predetermined correcting inverse function X C which is proportional to {U A -E 0} N+3/2+δ, wherein δ is a value included in a range extending from δ=-0.2 to δ=+0.2 in a vicinity of the coordinate X 0, namely in a spatial range satisfying the condition of 0≤U A -E 0 ≤0.1, and which is expressed as a smooth function continuing from the coordinate X 0 into the deeper region. with X denoting a coordinate along a central axis of the ion reflector, the reflector driver applies a voltage to the ion reflector so as to create, inside an inner hollow area of the ion reflector and along the central axis of the ion reflector, a predetermined potential distribution U A in which the potential monotonously changes over the entire ion reflector and therefore an inverse function X A can be uniquely obtained, thus creating an N-th order focusing position at a position with coordinate X 0 and potential E 0 inside the ion reflector, the N-th order focusing position being a position on the central axis at which a potential value is equal to the energy at which the first through N-th order derivative values of a total time of flight T of the ions with respect to an initial energy E of the ions are equal to zero; and the reflector driver also applies a voltage to the ion reflector within a space having the N-th order focusing position with coordinate X 0 as a starting point and extending into a deeper region, so as to superpose, on the predetermined inverse function X A, a predetermined correcting inverse function X C which is proportional to {U A -E 0} N+3/2+δ, wherein δ is a value included in a range extending from δ=-0.2 to δ=+0.2 in a vicinity of the coordinate X 0, namely in a spatial range satisfying the condition of 0≤U A -E 0 ≤0.1, and which is expressed as a smooth function continuing from the coordinate X 0 into the deeper region.