A submarine crew in the Republic of the Korea Navy experienced significant physical inactivity during undersea deployment because of the narrow and confined space. Physical inactivity is known to be associated with a number of adverse health conditions in the long-term perspective. This study aimed to assess the ambulatory activity of submarine crew using pedometers. Study subjects (n=109) were the submarine crew from two diesel submarines and personnel from the Submarine Command. The subjects wore pedometers at their waistline and recorded their walking steps daily for a month. The submarine crew walked more than 7000 steps/day on average during the stationed period. However, the ambulatory activity of the submarine crew greatly declined to a level of around 2000 steps/day during deployment, which corresponded to the sedentary status category. Active exercise is recommended for the submarine crew to prevent potential adverse health outcomes related to the physical inactivity.