We examined the effect of juvenile density on offspring performance of Neopallodes inermis Reitter (Coleoptera: Nitidulidae) in relation to the use patterns of mushrooms of Collybia spp. In total, 407 individuals from eight insect families were reared from 80 mushrooms from 1999 to 2001. N. inermis and Hirtodrosophila alboralis Momma et Takada (Diptera: Drosophilidae) were dominant. The emergence of N. inermis peaked at 20 d after mushroom collection, whereas that of H. alboralis peaked at 12 d. Adult females of N. inermis lay eggs soon after the mushrooms occur. The mean fresh mass of the mushrooms soon after their appearance from which N. inermis emerged was larger than that of mushrooms with no emergents. An increase in the density of N. inermis was associated with a shorter elytral length of N. inermis both in 1999 and 2000. However, a greater density of other insects was associated with a shorter elytral length only in 2000. In 2000, when H. alboralis was most abundant in the insect community, an increase in their density was not significantly associated with a shorter elytral length of N. inermis. This suggests that the effect of intraspecific competition could work consistently. Females of N. inermis possibly reduce the effects of competition by ovipositing on large immature mushrooms because a large resource patch reduces competition among insects by reducing their density, although it was not clearly indicated because of unpredictable factors acting on mushrooms.