A seal structure is provided to block a clearance between a rotational casing and a fixed housing for driving a crawler of a crawler vehicle. A seal ring is held in a first annular holding groove provided in the fixed housing together with a first O-ring that biases the seal ring toward the outside of the first holding groove. A second holding groove having an opening facing the first holding groove is provided in the rotational casing to hold a slide ring having a higher rigidity than that of the seal ring together with a second O-ring that biases the slide ring toward the seal ring. A foreign object such as mud is thereby prevented from invading a gear chamber without increasing a frictional loss.