A positive photosensitive siloxane composition having high photosensitivity and having such properties as high heat resistance, high transparency and low dielectric constant may be used to form a planarization film for a TFT substrate, an interlayer dielectrics or a core or cladding of an optical waveguide. The positive photosensitive siloxane composition includes a siloxane polymer, quinonediazide compound and solvent, and a cured film formed of the composition has a light transmittance per 3 .mu.m of film thickness at a wavelength of 400 nm of 95% or more.