A method of efficiently manufacturing a closed structural member by using a reduced number of parts and manufacturing processes for the closed structural member. In the method of manufacturing the closed structural member, a punch (84) is driven in the pressing direction to bend one joint end section (19) of a blank (24) by a pressing force applied from a flange working section (102). This forms said joint end section into an engaging flange section (20) which is superposed on the outer side of the other joint end section (21) to restrain the other joint end section (21). At the same time as above, the blank (24) is pressed by a pair of press-forming surfaces (96, 97) to form a pair of shoulder sections (26) of the blank (24) into predetermined shapes, respectively. Thus, said joint end section (19) is formed into the engaging flange section (20) which restrains the other joint end section (21) and is affixed to the other joint end section (21), and at the same time as above, the pair of shoulder sections (26) is press-formed into the predetermined shapes.