A skin gas collection tool includes a casing forming a tubular opening section, a tubular flexible film member arranged along the inner lateral surface of the casing and a gas collection vessel gastightly linked to the casing. The tubular flexible film member is held at the opposite ends thereof in gastight contact with the inner lateral surface of the casing to form a sealed space between itself and the inner lateral surface of the casing and forced to expand toward the center of the tube as a fluid substance is fed into the sealed space. The tubular flexible film member has a thickness and an elasticity satisfying a predetermined relation.
