The insect species richness of each island in the south-western Sea of Korea was considered on the background of the equilibrium theory. The species number of insects on Kwanmae Island in the present study (140 species) was much higher compared with a previous survey. Based on a literature survey of island biota surveys published in the 1980s, of 47 islands, the lowest species number (12 species) was on Kwanmae Island, and the most diverse insect fauna (254 species) was on Baekryong Island. The mean species number of surveyed islands was approximately 54 (53.96 with a standard deviation of 46.95). The median species number was 38 with a skew of +2.56. Insects, including the orders Odonanta, Orthoptera, Hemiptera, Lepidoptera, Coleoptera, Hymenoptera and Diptera, occurred on 32 of the 47 surveyed islands, and had an occurrence rate of more than 0.68. This indicates that these insects are distributed widely on the islands in the south-western Sea of Korea. The species number showed a significant linear relationship with both area of an island and its distance from the mainland (P < 0.05), with an extremely low determinant coefficient (r2 = 0.13 for area vs species number and r2 = 0.28 for distance vs species number). Other factors tested in the study failed to show a significant relationship with species number. A multiple-regression model established using area and distance as independent variables showed significant relationship with species number, with a relatively higher determinant coefficient (r2 = 0.70, P < 0.05). We present possible explanations to explain the difference between estimated and observed species number in Kwanmae Island.