1. A non-transitory storage medium containing program instructions that, when executed by a computer processor, cause the computer processor to perform a method, the method comprising: acquiring a first image and a second image that are captured by imaging a real marker with a camera when an inertial sensor is respectively at a first pose and a second pose, the camera and the inertial sensor being included in a head-mounted display device; acquiring a difference in pose between the first pose and the second pose based on output of the inertial sensor; and deriving a spatial relationship between the camera and the inertial sensor using at least the first image, the second image and the difference in pose, wherein the spatial relationship is derived by: calculating a rotational relationship between the camera and the real marker using the first image and the second image, and calculating a rotational matrix using the calculated rotational relationship and the difference in pose such that the rotational matrix is used to convert a coordinate system fixed to the camera into a coordinate system fixed to the inertial sensor.