A maximum number of charges on aerosol particles which are measurement targets is decided as a natural number x equal to or larger than 2. First and second electrical mobility groups are derived based on x. The first group includes electrical mobility Zc(U) and electrical mobilities having voltage values equal to a voltage U multiplied by values from 2 to x respectively, and the second group includes electrical mobilities having respective voltage values equal to voltage U multiplied by irreducible fractions which are coprime to each other among values with regularity. A ratio of flow rate with which range corresponding to the electrical mobilities included in the first group do not interfere with one another and the range corresponding to the electrical mobilities included in the first group and range corresponding to the electrical mobilities included in the mobility group do not interfere with one another is calculated.