When using an oblique-viewing scope, the rotation of cylinder changes the optical axis of the endoscopic camera. Thus, the angle of rotation is required to estimate the viewing direction in addition to the posture of the cylinder. In a previous study, rotary encoder is used for acquisition of the rotational angle of the cylinder. However, the rotary encoder tends to be cumbersome in clinical use, because the encoder must be attached between a cylinder and a camera head. In this study, we estimate rotation angle of the cylinder by using an accelerometer. The accelerometer detects the orientation of gravity. The rotation of the cylinder is calculated from the change of the orientation of the gravity. Besides, we considered the influence of movement of the endoscope on the accelerometer and executed movement decision. The result of experiments indicated that the method enabled to estimate the rotation angle within about 3 degrees of errors when the endoscope is inclined at an angle of less than 45 degree against the horizontal plane.