Image sampling of megafauna using a towed camera is immensely beneficial in deep-sea biological investigations because of the benefits of a wide investigation range, long duration, and no risk of specimen destruction. However, its automatic identification and error analysis are still understudied. A deep-sea megafauna body size measurement method was developed using towed camera observation photographs from the COMRA 50th cruise. We successfully obtained body size information of 12 sea cucumbers in the Pacific Ocean and observed a green giant sea cucumber 70.8 cm long. We examined the towed-camera system, installation, and observation errors, which imply that nonparallel lasers cause the most significant errors. A technique for correcting the inaccuracy caused by the swing of the towed camera was developed. Based on the above results, it is beneficial to logically develop the technical requirements for the examination of deep-sea megafauna with a towed camera and to encourage extensive investigation of deep-sea benthic megafauna.