A three-dimensional resolution chart is used to adjust the position of a solid-state image detector 4 in a given image-forming optical system. A plurality of blocks 10a are provided on an upper surface of a resolution chart 10. Each block 10a has an adjusting pattern A-D depicted on a slant surface thereof. An image of each adjusting pattern A-D is formed-on the solid-state image detector 4 through a master lens 2. The contrast integration value F is calculated based on the image of the adjusting pattern at each of a plurality of designated areas hj (j=1 - - - n) on the slant surface of each block 10a provided on the resolution chart 10, to find out a specific position on each block where the contrast integration value F becomes maximum, thereby identifying the specific position as focus point of the master lens 2. The back focus of the solid-state image detector 4 is calculated based on thus obtained focus point. An optimum gradient (.alpha., .beta.) of the solid-state image detector 4 is calculated based on three-dimensional relationship between plural focus points of the master lens 2 detected on the plural blocks 10a. Finally, each positioning mechanism 11 adjusts each solid-state image detector 4 to the calculated optimum position or at the optimum gradient.