The invention relates to a reaction apparatus having an enhanced connection, and a fuel cell system and an electronic device that include such a reaction apparatus. A reaction apparatus (1) a reformer (4) in which a reforming reaction chamber (31) is formed, a CO remover in which a removing reaction chamber (35) where a chemical reaction is performed at a temperature lower than that in the reforming reaction chamber (31) is formed, and a connecting portion (6) having a communicating path that communicates between the reforming reaction chamber (31) and the removing reaction chamber (35). The reformer (4) and the CO remover (5) are arranged spaced apart from each other, at least one of which (4, 5) is configured by combining ceramic parts (11, 12) and metal components (15, 16), and the ceramic parts (11, 12) and the metal components (15, 16) are connected with connecting members (18, 20) interposed therebetween. The connecting members (18, 20) have thermal conductivities higher than those of the ceramic parts (11, 12).