To enable the complete prevention of water leakage by providing a lip seal for the inflow opening part of an inner case and pressing the tip part of the lip seal against the inner peripheral surface to be fitted of the inner case i.e., the outer periphery of an inlet.

The inflow opening part of an inner case 5 is formed in a square shape larger than the inner diameter of an inlet 3, and a groove 14 for installing a lip seal 13 is endlessly formed at all the inner surfaces of the inflow opening part of the inner case 5. The main body of the lip seal 13 is squarely formed so as to be fitted into the installing groove 14, and a tip-shaped sealing part 13b with a tapered cross section is circularly formed at the center part. The sealing part 13b is formed approximately in a truncated conical shape with a large-diameter inner-end side and a small-diameter tip part, and its inner surfaces are formed in a circular surface. With the sealing part 13b directed outward, the lip seal 13 is easily installed to the inner case 5 dividing a bottom wall 5b by fitting the installing end part at the outer periphery into the installing groove 14. By this, it is possible to reduce water leakage and improve the performance of a water meter.