A substrate for a liquid filter contains a polyolefin microporous membrane. A mean flow pore size dPP in a pore size distribution of the polyolefin microporous membrane measured by a half dry method according to gas-liquid phase substitution is from 1 nm to 20 nm. A mean flow pore size dLLP in a pore size distribution of the polyolefin microporous membrane measured by a half dry method according to liquid-liquid phase substitution is from 1 nm to 15 nm. A difference (dPP−dLLP) between the mean flow pore size dPP and the mean flow pore size dLLP is 12 nm or less, and a thickness of the polyolefin microporous membrane is from 4 to 25 μm.