In recent years, agricultural fields and houses have been damaged by invasion by masked palm civets (Paguma larvata) in Japan. We investigated the size and shape of gaps large enough for them to enter (4 adult masked palm civets weighing 2.8-4.0 kg) and their approach behavior by trial-and-error testing in experiment 1. The gaps were rectangular (20 cmx2.5-10 cm at 2.5 cm intervals), square (2.5x2.5 cm-10x10 cm at 2.5 cm intervals), square (8x8 cm-11x11 cm at 1 cm intervals), and circular (8-11 cm at 1 cm intervals). In experiment 2, we investigated the relationship between the size and shape of the gaps large enough to enter and body part sizes (6 subadults and 6 adults weighing 2.2-3.4 kg). Gap sizes were changed to 1 cm intervals. In experiment 1, masked palm civets could enter by 7.5x20 cm rectangular, 8 cm square, and 10 cm diameter circular gaps. Exploration of gaps included not only olfactory and optical contacts, but also insertion of the muzzle into gaps that were obviously too small to go through. When a masked palm civet entered a gap, they moved their joints limber. In experiment 2, individuals could enter by 6x20 cm rectangular, 8 cm square, and 9 cm diameter circular gaps. There is no significant difference in body part sizes between individuals that went through the 6x20 cm rectangular gap and individuals that did not. By contrast, differences in sizes of body parts, such as shoulder and hip width, affected the minimum size of square and circular gaps that civets could go through. Japanese-style houses have many gaps to adjust to humid climate. It is necessary to cover these gaps with heavy fine-mesh materials to prevent invasion by civets.