There is provided a fiber-reinforced composite material containing fibers having an average fiber diameter of 4 to 200 nm and a matrix material, the composite material having a visible light transmittance of 60% or more at a wavelength of 400 to 700 nm, which is a conversion value based on a thickness of 50 mum. A fiber-reinforced composite material composed of a matrix material and a fiber aggregate impregnated therewith is provided, in which when a segment length of a bright region corresponding to a pore region of the fiber aggregate is represented by L, which is obtained by statistical analysis of a unidirectional run-length image formed from a binary image obtained by binarization of a scanning electron microscopic image of the fiber aggregate, the total length of segments that satisfy L>=4.5 mum is 30% or less of the total analyzed length. A transparent multilayered sheet, a circuit board, and an optical waveguide are also provided which use a transparent substrate formed from this fiber-reinforced composite material.