A cooling method of a thick-gauge steel plate by spraying water from a plurality of spray nozzles on the top and bottom surfaces of the thick-gauge steel plate conveyed between the adjoining pairs of constraining rolls, each comprising a top roll and bottom roll, constraining and conveying the steel plate so as to efficiently cool the top and bottom surfaces of thick-gauge steel plate to secure symmetry of temperatures of the top and bottom surfaces and uniformity of temperature in the plate width direction and achieve improvement of flatness of thick-gauge steel plate and uniformity of quality.