This invention provides a positive photosensitive siloxane composition having high photosensitivity and having such properties as high heat resistance, high transparency and low dielectric constant, used to form a planarization film for a TFT substrate, an interlayer dielectrics or a core or cladding of an optical waveguide. It is a positive photosensitive siloxane composition comprising a siloxane polymer, quinonediazide compound and solvent, characterized in that the light transmittance of the cured film formed of the composition per 3 .mu.m of film thickness at a wavelength of 400 nm is 95% or more.