Anchor device for seat belts, which is capable of greatly widening the range of regulation of the height of a slip anchor, designing a slide anchor of small weight and reducing the weight and dimensions of the anchor device. A slide anchor (19) is fixed to an elongated rail member (11) attached to a chassis-fixing portion so that the height of the slide anchor (19) can be regulated in the lengthwise direction thereof. Anchor bolt (25) projecting into the interior of a vehicle body is secured to the slide anchor (19), and a slip anchor (23) supporting an intermediate portion of a seat belt (80) is rotatably supported on the anchor bolt (25). A locking pin (27) is withdrawably passed through a bore (26) made through the anchor bolt (25), and a plurality of locking holes (12) are provided in the portions of the rail member (11) which are opposed to an end of the locking pin (27) so that the locking holes (12) are aligned in the lengthwise direction of the rail member (11). A spring (30) urging the locking pin (27) in the direction in which the end portion of the locking pin (27) is fitted in a locking hole (12) is provided thereon, and an operating portion (31) for withdrawing the locking pin (27) from a locking hole (12) is provided on the head portion thereof.