An index detection unit (110) detects the image coordinates of indices from a captured image. An index allocation information updating unit (160) calculates the position and orientation of an image capturing apparatus using the image coordinates of the indices and allocation information of each of these indices. Furthermore, the index allocation information updating unit (160) re-calibrates allocation information of an unreliable index having a reliability indicating that the allocation information is unreliable. The index allocation information updating unit (160) updates allocation information held by an allocation information holding unit (140) in association with the unreliable index to the re-calibrated allocation information and a reliability indicating that the allocation information is reliable.