NOVELTY - A base material has a hardened material layer formed using a resin composition and surface-treated by irradiating vacuum ultraviolet ray, on the surface. The resin composition contains composite resin formed by coupling silicon-containing structural unit (I) and/or (II), polysiloxane segment having silanol group and/or hydrolyzable silyl group, and vinyl polymer segment through a coupling of formula (III).    USE - Base material used for light receiving surface side protection sheet for solar cell module (all claimed). Can also be used for transport equipment, building material, motor vehicle, domestic appliances, and industrial material.    ADVANTAGE - The base material has excellent antifouling property and abrasion resistance, and provides light receiving surface side protection sheet with excellent weather resistance and surface physical property. The solar cell module having the protection sheet has improved electric power generation efficiency.    DETAILED DESCRIPTION - A base material has a hardened material layer formed using a resin composition and surface-treated by irradiating vacuum ultraviolet ray, on the surface. The resin composition contains composite resin formed by coupling silicon-containing structural unit of formula: -O-Si(-R1)(-O-)-O- (I) and/or -O-Si(-R2)(-R3)-O- (II), polysiloxane segment having silanol group and/or hydrolyzable silyl group, and vinyl polymer segment through a coupling of formula (III).    R1-R3=group having one polymerizable double bond chosen from -R4-CH=CH2, -R4-C(CH3)=CH2, -R4-O-CO-C(CH3)=CH2 and -R4-O-CO-CH=CH2, 1-6C alkyl, 3-8C cycloalkyl, aryl, or 7-12C aralkyl;and    R4=single bond or 1-6C alkylene.    Portion of vinyl polymer segment comprises carbon atom in formula (III), and portion of polysiloxane segment comprises silicon atom coupled only with oxygen atom. INDEPENDENT CLAIMS are included for the following:    (1) light receiving surface side protection sheet, which is obtained by irradiating vacuum ultraviolet ray to the surface of hardened material layer and surface-treating;    (2) solar cell module, which has the protection sheet in front surface of light receiving side such that hardened material is made into outermost layer; and    (3) surface treatment of hardened material layer.