A pipe expansion method capable of reducing an inspection range (area) of a heat-transfer pipe secured to a pipe plate and capable of shortening the time required for inspection is provided. In a pipe expansion method for securing a heat-transfer pipe inserted in a pipe hole of a pipe plate by expanding the pipe, after tightly fitting an outer circumferential surface of the heat-transfer pipe to an inner circumferential surface of the pipe hole from a primary-side end face to a secondary-side end face of the pipe plate, surface pressure between the heat-transfer pipe and the pipe plate is further increased in a predetermined range from the secondary-side end face, or close to the secondary-side end face, towards the primary-side end face.