A device for colorimetric assay of at least one component in a liquid sample, which device comprises a support having a through hole with an area of from 3 to 80 mm.sup.2, a transparent porous film which is provided on the support to cover the hole, a reagent layer provided on a surface of the porous film which is not contacted to the support, and a sample-holding layer which covers the reagent layer and a part of a surface of the support, whereby an amount or concentration of at least one component in the liquid sample may be easily and accurately measured.