One example of conventional decorative sheets is such that a protective layer formed of an ionizing radiation-cured resin, such as an electron beam-cured resin, is provided. In this type of decorative sheets, a pattern, such as a woodgrain pattern, is provided on the surface of a substrate sheet, such as paper, an olefin resin, or a vinyl chloride resin, and a protective layer formed of an electron beam-cured resin or the like is provided on the surface of the pattern. In general, such decorative sheets are applied, for example, as interior materials for fittings or buildings, to particle boards, plywoods, plastic members, or other substrates for decorative plates, and then used as decorative plates.
In conventional decorative sheets provided with a protective layer formed of an ionizing radiation-cured resin, the ionizing radiation-cured resin has a regulated crosslink molecular weight. In general, molecular weight between crosslinks or average crosslink molecular weight is used as an index of the crosslink molecular weight. For conventional electron beam-cured resins, for example, the average molecular weight between crosslinks is set to 150 to 1,000, more preferably 200 to 1,000, still more preferably 250 to 800. In this case, the average molecular weight between crosslinks is determined by the following equation:Average molecular weight between crosslinks=molecular weight of whole resin (m)/number of crosslink points 
In this equation, the molecular weight of the whole resin is Σ (number of moles of each component incorporated x molecular weight of each component), and the number of crosslink points is Σ[2(number of functional groups in each component −1) x number of moles of each component].
When the average molecular weight between crosslinks exceeds 1,000, the whole resin is excessively soft. In this case, high hardness characteristic of electron beam-cured resins is lost, posing a problem of scratch resistance of the protective layer in its surface. Even though the average molecular weight between crosslinks is 250 to 800, which has been regarded as a preferred molecular weight range for the conventional protective layer, for example, a protective layer having an average molecular weight between crosslinks of about 800 close to the upper limit value poses a problem of resistance to staining. In this case, in a test on resistance to staining, a contaminant is likely to be left on the surface of the protective layer. On the other hand, when the average molecular weight between crosslinks is brought to not more than 200, the resistance of the protective layer to staining is significantly improved. The reason for this is that the hardness of the protective layer in its surface has been increased by virtue of an increase in proportion of the functional group to the molecular weight. This enables the amount of the contaminant left on the surface to be reduced. When the average molecular weight between crosslinks is not more than 200, the electron beam-curable resin coheres at the time of curing, unfavorably leading to a failure of the print layer of a woodgrain pattern or the like underlying the protective layer. Consequently, a failure of adhesion occurs. In particular, when the ink composition constituting the print layer contains a large amount of a pigment, the separation between layer is likely to occur.
The present invention has been made with a view to solving the above problems of the prior art, and it is an object of the present invention to provide a decorative material which has a protective layer possessing excellent resistance to staining and does not cause a problem of a failure of adhesion or the like.