A real time analysis in accordance with temperature change and highly sensitive detection are permitted. A flow passage device has a flow passage (1) and is able to heat a fluid in the flow passage. A state of the fluid in the flow passage is observable using light. The flow passage device includes a first member (2) including an observation surface (3) and an upper inner wall surface (4), and a second member (5) including a lower inner wall surface that opposes the upper inner wall surface (4) and that is part of inner walls of the flow passage. A heating resistor (6) having a reflective surface that reflects light is provided on the lower inner wall surface. The light reflected by the reflective surface passes through the upper inner wall surface (4) and the observation surface (3).