The most annoying problem relating to the sensitivity of mask inspection systems is the encountering of false signals arising from nuisance defects. In order to overcome this problem, we have previously proposed a new algorithm for die-to-wafer-like image (D-to-WI) in real time. This paper describes the optimum mask inspection optics for the D-to-WI mask inspection. We examine the optimum mask inspection optics with numerical simulation for various numerical apertures (NAs) and partial coherence factors (σ) in these optics. The simulated result shows that the optimum mask inspection optics for the D-to-WI mask inspection has NA 0.9 and σ = 0.95 for an ArF-6%-phase shift mask (PSM) 260/260 nm line/space pattern on the mask plane, in which the 193 nm-ArF scanner has NA 0.93, σ = 0.92 − 0.92/0.3 annular illumination, reduction factor of ×4, and circular polarization incident light.