Abstract: The invention relates to a thin film solar cell including a transparent substrate, a transparent electrode layer, at least one photoelectric conversion unit, and a back electrode layer in this order from the light incident side. The transparent substrate includes a transparent base, a transparent undercoat layer having fine particles and a binder, and an insulating irregularity layer in this order from the light incident side. Consequently, light reflection by the transparent substrate is suppressed and optical path length of the incident light is increased due to light diffusion, so that improved optical confinement effect can be achieved.