A method for continuously measuring the surface temperature of a heated steel strip, includes providing a flat reflecting plate so as to face a heated steel strip at an angle of inclination (.alpha.) with the steel strip. A radiation thermometer measures the amount of heat radiation energy which is emitted from an arbitrary point on the surface of the steel strip and comes directly into the radiation thermometer; and the thermometer also measures the total sum of heat radiation energy which (a) is emitted from a different point on the surface of the steel strip and comes into the radiation thermometer after having been reflected at least twice between the steel strip and the reflecting plate and, (b) is emitted from a final reflecting point, on the steel strip, of the heat radiation from said different point. The emissivity of the steel strip is computed on the basis of said total sum of the energies of the heat radiations and the amount of energy of the heat radiation from the arbitrary point; and the surface temperature of the steel strip is measured on the basis of the computed emissivity and the amount of energy of a reference heat radiation. The final angle of reflection (.theta.) from the steel strip of the heat radiation from said different point, and the angle of inclination (.alpha.) of the reflecting plate, are set at values which satisfy specific limits.