To provide a sanitary sewage decomposition treatment device for ships, which effectively purifies sanitary sewage ejected from a toilet of a ship etc., while preventing environmental contamination with a simple and compact configuration.

The device includes: a septic tank 2 including a plurality of purification processing chambers 7 to 11 for gradually purifying sanitary sewage ejected from a toilet bowl 1 installed in a ship; a surplus water storage tank 3 for storing sanitary sewage purified by the septic tank 2, as surplus water; a water supply means 13 for supplying the sanitary sewage purified by the septic tank 2, to a water supply section for ships. Capacity of the surplus water storage tank 3 is set to a value corresponding to a waste amount of all crews ejected during a voyage period, according to the number of the crews and the voyage period of the ship.