One cycle of loop orbit is formed by two identical time-focusing unit structures (T1 and T2). Each of the time-focusing unit structures (T1 and T2) has a time-focusing point (P1) at the injection side and a time-focusing point (P2) at the ejection side. Each of them also has an injection-side free flight space (11) with a length of L1 and an ejection-side free flight space (12) with a length of L1, respectively anterior and posterior to a basic ion optical element (10) for causing ions to fly along a substantially arc-shaped orbit. Another basic ion optical element (30) having the same configuration as that of the basic ion optical element (10) is inserted to the injection-side free flight space (11) so that the distance between the ejection end of the basic ion optical element (30) and the injection end of the basic ion optical element (10) is L1′. The length L0 of the free flight space for injecting ions to the basic ion optical element (30) is set to be the value obtained by L0=2(L1+L2)−(L1′+L2). Accordingly, ions that depart from the starting point (Ps) are time-focused when they arrive at the time-focusing point (P2).