A position/orientation measurement method for calculating the position and orientation of a first image sensing unit, comprising: a first image acquisition step of acquiring a first image obtained by capturing a scene of a real space in which a first indicator (Q1, Q2, Q3) is arranged, by the first image sensing unit; a second image acquisition step of acquiring a second image including a second indicator (P1, P2) attached to the first image sensing unit, captured by a second image sensing unit fixed on the real space; a first detection step of detecting an image coordinate of the first indicator from the first image; a second detection step of detecting an image coordinate of the second indicator from the second image; and a position/orientation calculation step of calculating the position and orientation of the first image sensing unit using the image coordinate of the first indicator detected from the first image captured by the first image sensing unit in the first detection step and the image coordinate of the second indicator detected from the second image captured by the second image sensing unit in the second detection step, characterized in that: said position/orientation calculation step includes: a step of calculating a line that constrains a position of the second indicator attached to the first image sensing unit in the real space on the basis of the image coordinate of the second indicator detected in the second detection step and a position and orientation of the second image sensing unit, and a step of calculating the position and orientation of the first image sensing unit on the basis of the image coordinate of the first indicator detected in the first detection step under a constraint that the position of the second indicator is present on the line. a first image acquisition step of acquiring a first image obtained by capturing a scene of a real space in which a first indicator (Q1, Q2, Q3) is arranged, by the first image sensing unit; a second image acquisition step of acquiring a second image including a second indicator (P1, P2) attached to the first image sensing unit, captured by a second image sensing unit fixed on the real space; a first detection step of detecting an image coordinate of the first indicator from the first image; a second detection step of detecting an image coordinate of the second indicator from the second image; and a position/orientation calculation step of calculating the position and orientation of the first image sensing unit using the image coordinate of the first indicator detected from the first image captured by the first image sensing unit in the first detection step and the image coordinate of the second indicator detected from the second image captured by the second image sensing unit in the second detection step, characterized in that: said position/orientation calculation step includes: a step of calculating a line that constrains a position of the second indicator attached to the first image sensing unit in the real space on the basis of the image coordinate of the second indicator detected in the second detection step and a position and orientation of the second image sensing unit, and a step of calculating the position and orientation of the first image sensing unit on the basis of the image coordinate of the first indicator detected in the first detection step under a constraint that the position of the second indicator is present on the line. a step of calculating a line that constrains a position of the second indicator attached to the first image sensing unit in the real space on the basis of the image coordinate of the second indicator detected in the second detection step and a position and orientation of the second image sensing unit, and a step of calculating the position and orientation of the first image sensing unit on the basis of the image coordinate of the first indicator detected in the first detection step under a constraint that the position of the second indicator is present on the line.