A sensor device for measuring mixture concentration aims at accurately detecting concentrations of mixtures in a fluid. An internal electrode (23) is disposed on the inner side of an external electrode (22) coaxially with the external electrode (22), and a plurality of points in the length direction of the internal electrode (23) are supported by a casing (21) via supporting members (28a, 28b). The state of a detected fluid (S) passing through a passage (26) between the external electrode (22) and the internal electrode (23) is detected based on electrostatic capacitance between the external electrode (22) and the internal electrode (23).