1. A marker placement information estimating method for estimating placement information of a plurality of types of markers, the marker placement information estimating method comprising: using a processor to carry out the method of: a marker detecting step for detecting a plurality of types of markers, of which the placement information has different amounts of information, from multiple images in which scenes including markers are photographed, the plurality of types of markers including a first marker type of which the placement information is described with the position of one point within 3-D space, and a second marker type of which the placement information is described with the position and orientation within 3-D space; a marker projected position calculating step for calculating a projected position where a marker is projected on an image surface based on an approximate value of the position and orientation of an imaging unit when the multiple images were photographed and an approximate value of the placement information of each type of marker; a correction value calculating step for obtaining a correction value as to the placement information of each type of marker so as to reduce a sum of error between the position of each type of marker on the image surface detected in the marker detecting step and the projected position of the marker on the image surface obtained in the marker projected position calculating step; a placement information correcting step for correcting an arrangement of each type of marker based on the correction value obtained in the correction value calculating step; and a repeated calculating step for calculating the placement information of each type of marker by repeatedly performing the marker projected position calculating step, the correction value calculating step, and the placement information correcting step using the placement information of the marker corrected in the placement information correcting step instead of the approximate value of the marker projected position calculating step.