The purpose of this paper is to consider the fishing mechanism of the pot trap. Fish behavior was observed in and around the trap using underwater CCD cameras in the sea and in an experimental tank. Two types of underwater observation wers conducted at the fishing ground and open air experimental tank. Total of observation time was 3240 minutes in field experiments and 1440 minutes in experimental tank. Authors examined the puffer behavior in experiments and considered the fishing mechanism of the pot trap. The results showed that the catching ratio was 2%. The posture of fish while approaching the trap entrance were different between the egress and those caught. The snout was always pointed downward for egress. In captured fish, the snout direction changed upward rapidly. It was determined that a fish was captured when 0.4-0.6 of the length of its body passed through the trap entrance.