NOVELTY - The sheet (7) has a structure that is arranged in parallel between a pair of sheet portions such that the sheet portions arranges a prism on a surface so that a surface inserts sheet portions as a diffusion plane. The prism is provided with a height of 1/2 to 1/3 of a sheet thickness. The structure is arranged at equal intervals with respect to an installation short side of a solar cell module (1) at a parallel and a straight form on sheet surface. The microparticles consist of inorganic substances such as titanium oxide, magnesium oxide, barium sulfate, and silica.    USE - Highly reflective sheet for installation between solar cell modules such as bifacial solar cell module in solar cell module system (claimed) such as double-sided light reception type solar cell module system.    ADVANTAGE - The structure is arranged in parallel between a pair of sheet portions and the sheet portions arranges a prism on a surface such that the prism is provided with directivity to the sheet surface in multiple rows and distributed at equal intervals at reflected light at parallel and straight forms so that a surface inserts sheet portions as a diffusion plane so as to carry out uniform distribution as scattering reflector and diffusion filler to sheet base material and to reflect without sealing the incident light to sheet in the base material. Thus, the reflected light is efficiently reincident on prism at a cell light-receiving surface through a back surface glass board material and sealing material toward module back surface side while suppressing the age-deterioration such as reflectance fall by base-material modification accompanying solar radiation exposure, and delamination by contraction by setting the base material as the structure of inorganic substance main case.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a solar cell module system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the double-sided light reception type solar cell module system with highly reflective sheet. (Drawing includes non-English language text)    Solar cell module (1)    Glass (1a)    Cell (1c)    Surface incident light (5)    Highly reflective sheet (7)