The existence ratio of optical isomers (D-form, L-form) is easily measured.Counterclockwise or clockwise circularly polarized light is applied from a light application unit 6 to ions which are derived from a target compound and are drifting in a drift region 3. The ions absorb light, and a collision cross-section of the ions thus changes, whereby the mobility also changes. Each of the D-form and the L-form has a light absorption rate which is different between counterclockwise circularly polarized light and clockwise circularly polarized light. Therefore, each light absorption rate is examined in advance, and an existence ratio of the D-form and the L-form is calculated, based on the two intensity ratios each of which is the ratio of a peak of the ions having absorbed no light and a peak of the ions having absorbed light, which peaks appear on a drift time spectrum, and based on the light absorption rates of the D-form and the L-form.