MPS method is a sort of particle method (not a stencil computation) used for computational fluid dynamics. "Search of neighbor-particle" is a main bottleneck of MPS. We show our porting efforts and three optimizations of search of neighbor-particle by using OpenACC. We evaluate our implementations on Tesla K20c, GeForce GTX 1080, and Tesla P100 GPUs. It achieved 45.7x, 96.8x, and 126.1x times speedup compared with single-thread Ivy-bridge CPU.