A connecting construction of a branch pipe is disclosed. The construction includes a branch-pipe joint fixed to an outer peripheral face of an existing pipe and attached to a passage forming member for forming, between the passage forming member and the outer peripheral face of the existing pipe, a guide passage extending from a lower portion to an upper portion of the existing pipe along a peripheral direction of the pipe, with the branch-pipe joint being oriented along a radial direction of the existing pipe so that a branching pipe portion thereof for branch-pipe connection is opened at an upper portion within the guide passage, and a closing member for closing an upper through hole after formation of a lower through hole, the upper through hole being provided in an upper pipe wall of the existing pipe for forming, in a lower pipe wall of the existing pipe, the lower through hole communicated with a lower portion inside the guide passage through the branching pipe portion, with the closure of the upper through hole, communication being established between a branch pipe connected to the branching pipe portion and the existing pipe through the guide passage and the lower through hole. The closing member is fixed to the branching pipe portion. A method of connecting a branch pipe is also disclosed.