This paper presents the determination of number of cracks, lengths, locations and angles of plural cracks by inverse analysis on the basis of strains measured around the cracks. The strains due to plural cracks at any points in infinite plate are calculated by using the body force method. The analysis of inverse problem is carried out by using the PSO algorithm, in which the fitness is a reciprocal of difference between the strains calculated by the body force method and the measured strains. The effectiveness of present method is clarified through numerical simulation. The results show that the present method using the PSO algorithm and the body force method gives accurate results for inverse problem of plural cracks.