A shaft seal device is provided in which movement of the seal lip is prevented and the sealing function can be reliably maintained by shaping the adapter so that the adapter is more readily pressed into and less liable to be dislodged from the cartridge, and by increasing the width in the radial direction. An external peripheral-side flange of the adapter fitted into the cartridge on the sealing fluid side of the first lip seal member is formed in a small-diameter tapered shape toward the web side, and the web is composed of an external-periphery-side portion having an orthogonal planar shape that is orthogonal to the center of the shaft, and a tapered internal-periphery-side portion that slopes to the sealing fluid side.