To design three-mirror telescope system (F/8, 120 inch in focal length) for visible and infra-red band imaging, methods for power configuring and correction of the third order aberrations were studied. In the design of the telescope system, a three-mirror system corrected for spherical aberration, coma, and astigmatism was used for infra-red imaging, and the aberrations were corrected by using conic surfaces. For visible imaging, a singlet corrector lens was appended at the front of the focal plane to correct filed curvature. The telescope system has diffraction limited performance for 10 ${mu}{ extrm}{m}$ in wavelength within 2.4$^{circ}$ of field-of-view. In the visible band imaging, the rms spot size of the telescope system is less than 25 ${mu}{ extrm}{m}$ within 3$^{circ}$ of field-of-view for monochromatic light, and the telescope system satisfies flat field condition for CCD application.