We are constructing Mid-IR Test Observation System (MIRTOS) as an IR imaging system to evaluate and monitor the performance of the Japanese National Large Telescope, SUBARU. The system combines two cameras. One of the camera is for near-IR (NIR) and the other for mid-IR (MIR). They capture images simultaneously at the rate fast enough to freeze the seeing. Simultaneous NIR images are useful not only for evaluation of the image quality of the telescope but for two-wavelength shift-and-add that preserves diffraction-limited angular resolution of MIR images for a long integration. The system also has telescope emissivity mapping mode that images the telescope entrance pupil in MIR. For the MIR camera, a Santa Barbara Research Center (SBRC) Si:As IBS array with 320 by 240 pixels is used with pixel scale of 0.067 arcsec/pixel that takes enough samples to make diffraction limited images at 8 microns. For the NIR camera, an SBRC InSb array with 256 by 256 is used. Pixel scale is 0.028 arcsec/pixel. It is optimized to detect position of the brightest speckle in images at the wavelength of 2.2 microns with wide enough field of view to image a reference point source. In the emissivity mapping mode, a temperature controlled black body is inserted just outside the dewar window as an absolute calibration source for the telescope emissivity. In this paper we will present detailed design of MIRTOS.