Japan Atomic Energy Research Institute has performed the research and development of hydrogen production using the high-temperature engineering test reactor (HTTR). One of the key issues for the HTTR hydrogen production system is the development of control technology for stable operation. A thermal load absorber concept using a steam generator installed downstream of a reformer is proposed to mitigate a variation of helium temperature. Thermal-hydraulic analyses for the start-up operation and the suspension of the feed gas supply to the reformer are carried out. These results show that a large variation of the reformer outlet helium temperature takes place because of a change of the feed gas flow rate. However, the steam generator can mitigate the variation of the helium temperature. It is clarified that the HTTR can continue normal operation independently of the feed gas flow rate.