The invention relates to a reaction apparatus that efficiently heats a reaction portion, and a fuel cell system and an electronic device that include such a reaction apparatus. A reaction apparatus (1) includes a reformer (4) in which formed are a reforming reaction chamber (31) and a reformer combustion chamber (30) that generates heat to be supplied to the reforming reaction chamber (31), which (30, 31) are adjacent to each other with a partition interposed therebetween; a CO remover (5) in which formed are a removing reaction chamber (35) where a chemical reaction is performed at a temperature lower than that in the reforming reaction chamber (31) and a remover combustion chamber (34) that generates heat to be supplied to the removing reaction chamber (35), which (34, 35) are adjacent to each other with a partition interposed therebetween; and a connecting portion (6) that connects the reformer (4) and the remover (5). At least one of the reformer (4) and the CO remover (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 second members (18, 20) interposed therebetween.