PURPOSE: To reduce the traveling resistance and prolong the endurance life of a horizontal shaft roller by axially supporting a bearing member on the horizontal shaft roller for back up and supporting the horizontal shaft roller for back up at an eccentric position in the longitudinal direction for a vertical shaft of each bearing member.

CONSTITUTION: Brackets 12a and 12b are installed on an electric car frame 1a, and bearing members 14a and 14b are supported around the vertical shafts 13a and 13b on the upper side on the horizontal part. On the bearing members 14a and 14b, the horizontal shaft rollers 6a and 6b for back up are supported at the position in deflection in the longitudinal direction for the vertical shafts 13a and 13b by the horizontal shafts 15a and 15b. Accordingly, the horizontal shaft rollers 6a and 6b for back up contact the undersurface of the horizontal rail part 2a on the underside of a guide rail 2, and the seesaw motion of a transport electric car 1 around a driving wheel 3 is prevented. Further, since each horizontal shaft roller 6a, 6b can be steering-revolved, it can be set parallel to the traveling direction.