A seal structure of a crawler-driving apparatus, arranged between a rotational casing that drives a crawler of a crawler vehicle and a fixed housing fixed to a vehicle body of the crawler vehicle, characterized by: a first annular holding groove formed in one of the rotational casing and the fixed housing; a second annular holding groove formed in the other one of the rotational casing and the fixed housing to face the first annular holding groove; a seal ring held in the first holding groove; a slide ring that is held in the second annular holding groove and has a higher rigidity than that of the seal ring; a first elastic member (42, 47) held in the first annular holding groove to bias the seal ring towards the slide ring; and a second elastic member (41, 46) held in the second annular holding groove to bias the slide ring towards the seal ring, wherein one of the rotational casing and the fixed housing has an annular convex portion that protrudes into another one of the rotational casing and the fixed housing, has an end face, and is formed concentrically with a rotation axis of the rotational casing, the other one of the rotational casing and the fixed housing has an annular concave portion having a bottom surface to receive the annular convex portion, the first annular holding groove has an opening in the bottom surface of the annular concave portion, the second annular holding groove has an opening in the end face of the annular convex portion, a gap between the bottom surface of the annular concave portion and the end face of the annular convex portion located outward from the first and second annular holding grooves (45, 44) communicates to an outside of the rotational casing and the fixed housing through a radial gap formed between the rotational casing and the fixed housing in an outer circumference of the annular convex portion and an axial gap formed between the rotational casing and the fixed housing at an end of the radial gap, and a gap between the end face of the annular convex portion and the bottom surface of the annular concave portion located inward from the first and second annular holding grooves (45, 44) communicates to a gear chamber filled with lubricant and formed in the rotational casing through a radial gap formed between the rotational casing and fixed housing in an inner circumference of the annular convex portion. a first annular holding groove formed in one of the rotational casing and the fixed housing; a second annular holding groove formed in the other one of the rotational casing and the fixed housing to face the first annular holding groove; a seal ring held in the first holding groove; a slide ring that is held in the second annular holding groove and has a higher rigidity than that of the seal ring; a first elastic member (42, 47) held in the first annular holding groove to bias the seal ring towards the slide ring; and a second elastic member (41, 46) held in the second annular holding groove to bias the slide ring towards the seal ring, wherein one of the rotational casing and the fixed housing has an annular convex portion that protrudes into another one of the rotational casing and the fixed housing, has an end face, and is formed concentrically with a rotation axis of the rotational casing, the other one of the rotational casing and the fixed housing has an annular concave portion having a bottom surface to receive the annular convex portion, the first annular holding groove has an opening in the bottom surface of the annular concave portion, the second annular holding groove has an opening in the end face of the annular convex portion, a gap between the bottom surface of the annular concave portion and the end face of the annular convex portion located outward from the first and second annular holding grooves (45, 44) communicates to an outside of the rotational casing and the fixed housing through a radial gap formed between the rotational casing and the fixed housing in an outer circumference of the annular convex portion and an axial gap formed between the rotational casing and the fixed housing at an end of the radial gap, and a gap between the end face of the annular convex portion and the bottom surface of the annular concave portion located inward from the first and second annular holding grooves (45, 44) communicates to a gear chamber filled with lubricant and formed in the rotational casing through a radial gap formed between the rotational casing and fixed housing in an inner circumference of the annular convex portion.