To obtain an expansion measuring method measuring and correcting a change in the observation direction of a marker observation apparatus and detecting the relative positional relationship between markers with high accuracy.

A two-axis angle-of-inclination measuring means 6 is built in a high-rigidity frame 4 which measures a change in the observation direction of a marker observation apparatus. Two angle-of-inclination components which are obtained by the two-axis angle-of-inclination measuring means 6 and which express the posture of the high-rigidity frame 4 are measured. Distances up to two reference points 7a, 7b on the high-rigidity frame 4 are measured respectively from reference positions 9a, 9b which are formed outside an oven by using a distance measuring means 8. An angle-of-rotation component inside the horizontal plane, which expresses the posture of the high-rigidity frame 4 is computed on the basis of the difference between two measured distances and corrected. Thereby, the position of a marker is measured with good accuracy.