Maritime Staff Office
The staff office of the Japan Maritime Self-Defense Force (JMSDF), responsible for operations, planning, and administration.
The Maritime Staff Office oversees operations and training of the Japan Maritime Self-Defense Force.
海上自衛隊 is the Japan Maritime Self-Defense Force as a whole, while 海上幕僚監部 is its staff office.
幕僚監部 is a general term for a staff office in the Self-Defense Forces; 海上幕僚監部 is the maritime branch.
Compound of 海上 (maritime) + 幕僚 (staff) + 監部 (supervisory department). The term follows the naming pattern of the Self-Defense Forces' staff offices.