The present invention provides a technique which can disassemble a mandrel having a substantially cylindrical shape and being dividable into a plurality of segments, easily and efficiently. The disassembling method comprises, in the mandrel adhesively attached with a composite material structure on an outer peripheral surface thereof, a rotation step of rotating the pair of support rings along with the mandrel to position a segment which is a detached target to an uppermost portion; and a segment detaching step of detaching the segment which is the detached target positioned at the uppermost portion, from the pair of support rings; wherein in the segment detaching step, the segment which is the detached target is moved in a vertically downward direction to a position inside of the mandrel, between the pair of support rings, and is carried out from between the pair of support rings.