The disclosure provides a tightness adjusting device, including an adjustable rope, two toothed straps, and a slide fastener fitting with the two toothed straps, wherein each of the toothed straps includes a woven strap and a teeth row arranged on a work side of the woven strap, the slide fastener defines two passages therein, the two teeth rows stretch into the two passages respectively, the two passages are converged at least on a first end to form a tightening passage, the slide fastener is also provided with a separating block, the separating block is arranged between the two passages, the separating block is provided with a separating cone of which a tip end is toward the tightening passage, the separating cone is one-to-one corresponding to the tightening passage, and the adjustable rope is in connection with the slide fastener. With such arrangement, the tightness adjusting device provided in the disclosure can have the adjustable rope arranged in the interior completely, with a simple and compact structure; the adjustable rope in the interior can be tightened or loosened through pulling the slide fastener; in addition, when the tightness adjusting device provided in the disclosure is used to adjust tightness, it is only needed to pull the slide fastener, and single hand operation is enough.