PURPOSE: To provide an extension method for a shower bath unit to an existing service vessel capable of extending a shower and a toilet so easily in a short period of time.

CONSTITUTION: A shower bath unit 10 to be extended is completed in advance, and this shower bath unit 10 is fixed to the outside of an opening 3 formed in an outer wall 2 of each living room 1, and a series of works 26-29 including a plumbing pipe or the like are completed. With this constitution, a construction period in an existing service vessel is shortened while extension work takes place at the outside of each living room 1 and thereby work for the living rooms 1 is restricted to the opening 3 alone, thereby dispensing with removal and restoration works such as equipment or the like in each living room 1.