In AR surgery, a surgeon is supported by the 3D organ model that overlaid on the surgical view. 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, calibration is required since the cylinder makes a precession movement against ideal rotation axis. Besides, the rotary encoder attached to the endoscope tends to be big. In this study, we calculate the posture of the cylinder against the camera head by using optical marker and accelerometer. Accelerometer detects the orientation of gravity. The posture of the cylinder is calculated from the change of the orientation of gravity. The proposed method enables to estimate the viewing direction of oblique-viewing endoscope camera without calibration. In this presentation, we report the details of the proposed method and the result of AR images obtained by using our method.