An adjustment device has a frame, a guide, a holding bar, a connector, and a manipulating unit. The frame has a holding inclined surface. The guide has two guiding slots extending longitudinally. The holding bar is disposed in and is movable along the guiding slots. The holding bar and the holding inclined surface clamp the webbing of the seat belt. The connector is mounted around the frame and is connected to the webbing. The manipulating unit is slidably mounted around the connector and has a pushing portion formed on a back side of the front slat and disposed under the holding bar and in front of the holding inclined surface. The adjustment device is to connect the webbing's second segment that is positioned vertically and clamp the webbing's first segment that is fastened obliquely on a user, and is simply operated by stepless adjustment to enhance comfort and safety.