Virtual fluoroscopy is a surgical navigation technique that is often used in orthopedic surgery. In virtual fluoroscopy, a surgeon draws tool insertion lines on intra-operative X-ray images as a surgical plan, and the position of an actual surgical tool tracked by a position sensor is also drawn on the X-ray images. The surgeon can intuitively monitor the positioning error between the plan and the tool without continuous X-ray imaging; therefore, X-ray exposure is significantly reduced compared with that in conventional fluoroscopic procedures. However, a large navigation error is often reported even when the surgeon places the tool on the planned line. To address this issue, we investigated the reason for the error by using a mock virtual fluoroscopy system. From the results of the mock operation study, we revealed that the tool positioning error depends on the convergence angle around the plan path rather than the camera rotation angle.