PURPOSE: To minimize the time taken to advance legs by a method in which two sets of legs are plurally set in a vertically movable manner and a carriage well is formed by turning its one direction outwards in relation to the advancing direction and the other portion inwards.

CONSTITUTION: Legs 4 and 5 with carriages 3 are set within a carriage well 2 provided to a ship body 1. Of the paired legs 4 and 5, the carriage well 2 of the leg 4 is turned a slightly outwards at an angle +α in relation to the advancing direction and the carriage well 2 of the other leg 5 is turned inwards at an angle -α. In the walking, the ship body 1 is advanced while supporting it by one set of legs 4, legs 5 previously restored to the original positions are landed on the bottom of the ship, the load of the ship body 1 is transferred to the legs 5, and the legs 4 are drawn out, whereupon the ship body 1 can be soon advanced by the carriage 3 of the legs 5.