Disclosed is a technique of executing: a fixing step of coupling a pulling-raising rope 171 to a vicinity of a distal end 73 of a strut 70; a first raising step of pulling the pulling-raising rope 171 rearwardly to cause a swingable portion 78 of the strut 70 to swing upwardly and rearwardly; a second raising step of pulling the pulling-raising rope 171 rearwardly until a strut body portion 76 has a rearwardly inclined posture with respect to a vertical direction; a strut returning step of pulling a pulling-returning rope 23a forwardly to a position where a forward swinging movement of the swingable portion 78 with respect to the strut body portion 76 is restricted; and a coupling step of coupling the swingable portion 78 and the strut body portion 76 to each other in a non-swingable manner.