A coating liquid for forming porous silica coating, comprising a product of reaction between a short fiber silica and a hydrolyzate of an alkoxysilane of the formula XnSi(OR)4-n or a halogenated silane of the formula XnSiX′4-n (in the formula, X represents a hydrogen atom, a fluorine atom, an alkyl group having 1 to 8 carbon atoms, an aryl group or a vinyl group; R represents a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, an aryl group or a vinyl group; X′ represents a chlorine atom or a bromine atom; and n is an integer of 0 to 3). A coated substrate comprising a porous silica coating film formed from the above coating liquid for forming porous silica coating. A short fiber silica having an average diameter (D) of 10 to 30 nm, a length (L) of 30 to 100 nm and an aspect ratio (L/D) of 3 to 10. The above coating liquid for forming porous silica coating enables forming an insulating film which is excellent in adherence to a substrate surface, mechanical strength, chemical resistance and crack resistance and enables flattening irregularities of a substrate surface to a high degree. The coating film of the coated substrate has the above excellent properties.