This invention relates to a position/orientation measurement apparatus which can measure a position and orientation while achieving both high stability and precision. An image including indices laid out on a space is captured, and the indices are detected from the captured image. When a plurality of indices are detected, their distribution range is calculated, and an algorithm to be applied in position/orientation calculations is selected according to the size of the range (S3033, S3050, S3060). For example, when the indices are distributed over a sufficiently broad range, six parameters of the position and orientation of an image capture device are calculated as unknowns (S3070). As the distribution range of the indices becomes smaller, the degrees of freedom of unknown parameters to be calculated are reduced (S3035, S3025).