(57) A method of preheating steel strips in continuous heat-treatment using a heating zone including radiant tubes comprises a combination of first and second preheating stages. In the first preheating stage, visible heat contained in burned waste gas from the heating zone is heat-exchanged with a gas to recover part of the visible heat into the gas which is directed to a steel strip. In the second preheating stage, a heat medium is heated in a heating device and supplied and circulated into rolls. The steel strip is wound about the rolls to pass thereon in a non-oxidizing atmosphere. In this manner, the steel strip is preheated progressively to higher temperature to achieve high temperature preheating without causing serpentine movement of the steel strip, thereby compacting the heating installation and improving the production efficiency.