A hydrophobic treatment method is used in which at least one sheet filled with a gelled silicic acid is tilted at least two degrees with respect to a horizontal direction in a hydrophobizing solution, and the gelled silicic acid is hydrophobized. A manufacturing method for a sheet-like member is also used. The manufacturing method includes: a sol preparing step of adjusting the pH of a water glass aqueous solution to obtain a sol solution of silicic acid; an adding step of adding the sol solution to a fiber; a gel step of polymerizing and gelling the sol solution; a hydrophobic treatment step of hydrophobizing the gel with the hydrophobic treatment method; and a drying step of drying the hydrophobized gel.