To easily get over various obstacles of a stack shell by installing plural planetary wheels which are attracted to the shell by magnetic force and can respectively rotate and can integrally revolve, on a carriage.

Planetary wheel mechanisms 1 of four sets in total are longitudinally and laterally installed in a carriage. These planetary wheel mechanisms 1 are arranged at an interval of 120°, and have three planetary wheels adhered to a traveling surface by magnetic force and a carrier 3 to support the planetary wheels so as to be able to rotate and revolve. Normally, rotation of a sun gear 2 is transmitted to an output gear 5 through a planetary gear 4, and the output gear 5 and the planetary wheels rotate, and are put in a two-wheel traveling condition. On the other hand, the planetary wheels collide with an obstacle, and when its rotation is put in a substantially stopping condition, two wheels positioned on the traveling surface side rotate while sliding, or the output gear 5, the planetary gear 4, the sun gear 2 and the carrier 3 integrally rotate, and the three planetary wheels revolve as a whole, and can get over the obstacle.