Seasonality in life history and population dynamics of insect species is a very important attribute to be explored in depth for better understanding the dynamics of biodiversity and community structure in the tropical rainforests, because assemblage of insect species dominates the biodiversity and take various important roles in the community there and because the seasonality in the climatic conditions is rather weak there, compared with those in the other areas of different climate zones, where most insect species develop their own adaptations to the seasonal climatic changes. The problem whether or how insects living in tropical rainforests respond to such weak seasonal change there has been drawing attentions of many researchers studying ecology of tropical insects. To answer these questions, we analyzed the data of temporal change in insect abundance, obtained through the five-year monthly light-trapping, in a tropical rainforest in Borneo, where the climatic conditions are less seasonal, compared to the other tropical regions, and got the results that most insect species show notably weak seasonality in their population fluctuations. We will compare the results to those obtained from the insect assemblages in the other areas and discuss the implications for community ecology in tropical rainforests.