A theoretical and experimental study was conducted to investigate the critical condition at which a liquid drop slides down on an inclined plate. When the drop starts to slide down, it is suggested that the advancing and receding contact angle may appear on the circumference of the contact area of drop on the plate. The critical condition can be obtained from the force balance in the same way as the classical theory. In order to verify the validity of the condition, the occurrence of the sliding down is investigated when the plate on which a drop is set is tilted gradually from the horizon. The critical inclination angle of the plate φ_c is theoretically calculated by use of the drop profile obtained from the Laplace equation. In the experiment, φ_c was measured for each combination between 3 kinds of test liquids and test plates. The results agree well with calculated ones within the accuracy of the measurements.