Silica particles having a thiol group on a surface thereof, and satisfying the following conditions (a) to (c): (a) a particle diameter is 20 to 1,000 nm; (b) a density of the thiol group on the surface of the silica particles is 0.002 to 0.2 piece/nm 2 ; and (c) a ratio (B/A) in terms of an amount B (piece/particle) of the thiol group existing on the surface of the silica particles to an amount A of sulfur elements in the silica particles (the number of sulfur elements derived from thiol per silica particle) is 0.10 to 0.60.