In maintaining environments for plant growth effectively and properly, it is necessary to perceive and to evaluate optical radiant conditions not only in indoor areas but also under natural radiation. This is true especially under natural radiation, where optical radiant conditions change frequently as time passes. So it is desirable to be able to measure optical radiant conditions quickly, accurately and easily. In this paper, we report on an application of a spectroradiometer to measure the optical radiant conditions for plant growth. In this system, a fiber optic probe was used to improve the freedom and workability for measurements. A spectral dispersion system and a linear image sensor, which is inexpensive and easy to obtain because of recent developments in solid state technology, was utilized instead of a mechanical wavelength scanning system. The spectral dispersion system and a linear image sensor together are called multi-channel spectroradiometer. High speed measurement can be insured by its use under natural radiation. In designing this measurement system, two points were improved to reduce errors. One cosine-corrected the angular responsibility of the spectroradiometer for the measurement of spectral irradiance under natural radiation. The fiber probe of the spectro-radiometer is cosine-corrected by a shielding tube with a diffuser head. The other point is to provide an automatic wavelength scale calibration system using a 7 line spectrum. The combination and error reduction methods of these instruments contributed to the application of a high-speed and high-accuracy multi-channel spectroradiometer to measure the optical radiant conditions for plant growth.