The main function of work assistive suits is to reduce the burden on the muscles around the waist during stooping, thought to be the one of the causes of low back pain. Since fishermen frequently stoop, the introduction of work assistive suits is highly anticipated. In this study, the effect of a work assistive suit on the fish unloading task in a skipjack tuna pole and line fishing boat on offshore waters was experimentally investigated. A prototype work assistive suit, using carbon fiber-reinforced plastic plates as the elastic body of the suit, was manufactured and provided for the experiment. We measured the activity of the elector spine of four subjects using surface electromyography. The results showed that the muscle activities of the elector spine adjacent to the fourth lumbar vertebrate were significantly reduced in two subjects (the other two did not have successful experiments) and the rate of reduction ranged from 11% to 25%. The muscle activity in the elector spine adjacent to the first lumbar vertebrate was slightly increased in a subject, however, no significant changes were seen in other three subjects. It is expected that the work assistive suit considering actual work of fishermen is developed and disseminated.