The diversity of spider communities was investigated in the pine forest of Pinus densiflora in Songri-san, where chemical pesticides had been applied to control pine gall midge (Thecodiplosis japonensis). Spider communities were surveyed in four areas: a pesticide-untreated natural forest (area A), a forest with vinyl-covered ground surface (area B), an aldicarb-treated forest (area C) and a forest treated with a systemic pesticide (phosphamidon) (area D). A total of 74 spider species from 17 families were collected from the four survey areas. There were 54 species from 15 families in area A, 27 species from 12 families in area B, 29 species from 9 families in area C and 34 species from 9 families in area D, respectively. The species diversity of spider communities was highest in pesticide-untreated area A, and much lower in the other three areas. The monthly species diversity of spider communities was highest in May and lowest in January. The similarity of the spider communities was highest in areas B and D. The monthly similarity of the spider communities was highest in November and February. The dominant species was Clubiona jucunda (12.71%, N=304 individuals). According to our results, the application of aldicarb and phosphamidon dramatically decreased spider diversity and the number of individuals in the forests. Thus, the application of these two pesticides to natural forests should be restricted, or alternative pesticides need to be developed. Our results also indicated that application of the pesticides should be avoided in May and June when high spider diversity is expected.