A moving float type water-tube tiltmeter is designed for a reliable continuous record ing of secular ground tilt and earth tide observations. It is aimed in the present design that the detector should have such features as a wide dynamic range, a high resolution, an easy and secure calibration device, and very small mechanical drift. These are achieved by improving several defects associated with moving float type water-tube tiltmeters that have so far been developed. The diminishing effect, the difference between float movement and water level change, is markedly decreased by the adoption of a weak cross spring suspension for the fulcrum and a reduced mechanical magnification to decrease suction effect of the sensor. The diminishing factor in the present design, which is experimentally determined, is as small as less than 1%. A knife edge joint for connecting float rod to the arm can minimize the torque due to unbarance of the float metacenter which may cause spring creep or mechanical drift of the detector. Setting the hinge of calibration arm on the same axis of the fulcrum of the float arm, we can keep a constant sensitivity for a wide range, by which a highly reliable calibration is possible. Conversion coefficient for water level change has been obtained in a good ac curacy by this calibration mechanism. Although some errors (±2% at maximum) might be inherent in the case of absolute calibration, the relative accuracy between detectors has been found to be excellent under the routine observation.