In order to hold a long member without using a fixing jig and without deforming the long member in holding the long member, a long member assembling device is provided with: multiple hand parts for gripping a long member; and arm parts and trunk parts for moving the hand parts to adjust the positions of the hand parts gripping the long member. The hand parts have a configuration such that, when the positions thereof are adjusted by the arm parts and the trunk parts, the hand parts are capable of moving in the longitudinal direction of the long member while gripping the long member.