To provide a mortar substrate sheet capable of being easily executed without generating any separation or dry-out phenomenon of the mortar or corrosion of the substrate, and excellent in strength at a low cost.

In a mortar substrate sheet 11, a water-proof and moisture- permeable sheet 13, a water-absorptive and water-holding sheet 14, and a reinforcement sheet 15 to provide the prescribed strength to the mortar substrate sheet 11 are laminated from the substrate (back) side, and polyethylene which is 0.940g/cm3 and lower in density and 30μm and thinner in thickness is used in the water-proof sheet 13. A resin woven texture 12 which is elongated in a sheet and has porosity may be integratedly laminated on the back side of the water-proof sheet 13.