The invention provides a reaction apparatus capable of efficiently causing a reaction, and a fuel cell system and an electronic device that include such a reaction apparatus. A reaction apparatus (1) includes a reformer (4), a CO remover (5) and a connecting portion (6) that connects the reformer (4) and the remover (5). The reformer (4) has 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. The CO remover (5) has a removing reaction chamber (35) 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. The reformer (4) and the CO remover (5) are arranged spaced apart from each other, at least one of which is configured by combining ceramic parts (11, 12) and a reformer lid member (15) and a remover lid member (16), and heaters (48, 49) are disposed so as to face the ceramic parts (11, 12).