Boilers in coal-fired thermal power plants were often damaged by earthquakes such as the Great East Japan Earthquake in 2011. Since the coal-fired thermal power generation has been one of the main power generation methods after the Great East Japan Earthquake, mitigation of damage of boilers in thermal power plants by earthquakes is the very important subject in order to recover our daily life immediately after strong earthquakes. Meanwhile, a boiler in a coal-fired thermal power plant was damaged by Hokkaido Eastern Iburi Earthquake in 2018, and this damage was one of the causes of Hokkaido’s prefecture-wide blackout. According to a report by an electric power company, a damage occurred between a furnace and a cage of the boiler. In general, lengths, shapes, weights and so on of a furnace are different from a cage, so vibration characteristics and seismic response are different as well. Thus the connecting part between the furnace and the cage is a weak point in the boiler, and the damages often occurred there. Therefore this paper investigates seismic response of a boiler by a numerical analysis using a frame model from the viewpoint of the damage of the furnace and the cage. Various seismic waves were used as input waves in order to investigate the influence of the input wave. A result of a modal analysis was also provided in this paper.