The thrust load of an outer shaft (11) is received by a contra-rotating thrust bearing (40) provided in a boss (13a) of a front propeller (13) and transmitted to an inner shaft (12). The combined thrust load of the outer shaft (11) and the inner shaft (12) is received by an inner thrust bearing (41) provided on the bow side of the power transmission unit (20A) and transmitted to a ship body (2). The rotational force of an outer output shaft (24) is transmitted to the outer shaft (11) via a flexible joint (19) (gear coupling (19A)). Thus, the thrust load of the outer shaft (11) is transmitted to the ship body (2) only via the inner shaft (12) without being directly transmitted to the ship body (2), and the axial displacement and angular displacement are allowed by the gear coupling (19), so that it is possible to eliminate the influence of the change of hull deformation on the shaft center of the outer shaft (11).