Provided is a method of manufacturing a pipe including a large-diameter portion, a small-diameter portion, and a reduced diameter portion connecting the large-diameter portion and the small-diameter portion, the method comprising: forming a metal plate member into a U-shaped member having a U-shaped cross section; and forming the U-shaped member into a shape of the pipe. The U-shaped member includes: a curved portion facing an opening between both edges of the U-shaped cross section; and two facing side walls continued from the curved portion. An opening degree of the two facing side walls in a portion to form the small-diameter portion is larger than an opening degree of the two facing side walls in a portion to form the large-diameter portion.