IPCC (2013) warned that heavy precipitation events and annual precipitation would increase in mid-latitudes of the northern hemisphere in the future. However, it is little known how plant community responds to increasing rainfall, especially in eastern Asia with monsoonal climate including Japan. We investigated herbaceous plant community under rainfall treatments at semi-natural grassland in Hokkaido Japan. We established 30 plots and had manipulated the rainfall by using the rain-out shelter in growing season for four years; either drought (-50%), irrigation (+50%), or control. We evaluated how taxonomic and functional composition of each plot changed along with soil environmental variables by using seven functional traits. As a result, the soil water contents tended to increase under irrigation treatment. In post-treatment period, increase of the soil moisture led to decrease Shannon’s evenness, and increase functional diversity of each plot, suggesting being caused by competitive exclusion among similar species of functional traits such as plant height. Considering these patterns, the functional redundancy of species, which contributes to the community stability, tended to decrease at wet soil environment. Our results suggest that predicting the plant community would be more difficult due to increase of uncertainty through limiting similarity process along with increasing rainfall.