This paper proposed a model of capture process of trap for puffer, considering the behavior of puffer in the trap entrance. The model is based on the observation in the trap for puffer, which was conducted in the field and experimental tank. Fundamentalism in this model is analyzed by the probability (PC) that puffer pass the trap entrance. The probability (PC) is expressed using the geometric probability concerning the size of puffer (BL: cm) and entrance (EW: cm). The puffers with the size equal to or less than 0.5 BL/EW can swim backward, and these with the size over 2.3 BL/EW never do. The issues and the applications related to stock management were discussed.