To provide a thermal stencil printing base paper which enables attainment of a clear and high-quality image free from a void or nonuniformity of density even in a high resolution.

In the thermal stencil printing base paper constituted of a thermoplastic resin film and an ink passing porous substrate, a dispersion index of the reflected light of the substrate is 13 or above and the total sum of the area rates of a high basis weight region and a low basis weight region in 0.5 mm2 or above of the substrate is 3% or below, provided that the dispersion index=h/(L×100) where h is the maximum peak frequency and L the maximum class exceeding 500 frequencies-the minimum class exceeding 500 frequencies+1, as to a 64-class histogram of the image density of the reflected light obtained by taking in (10 cm)2 by 787 pixels. The high basis weight region is (the class of the maximum peak frequency+5 classes) or above, while the low basis weight region is (the class of the maximum peak frequency-5 classes) or below, and the total sum of the area rates (%)={(the total area of the high basis weight region of 0.5 mm2 or above+the total area of the low basis weight region of 0.5 mm2 or above)/(the area of the image taken in)}×100.