To provide an SOx concentration detection device that can detect the SOx concentration in exhaust gas without using a dedicated sensor. As shown in FIG. 4, the current value that flows when a voltage in the limiting current region is applied stays at a constant value (limiting current value) regardless of the elapsed time. On the other hand, the current value that flows when a voltage in the overcurrent region is applied decreases with the lapse of time and approaches the limiting current value 30 seconds after the start of application of the voltage. In this embodiment, the current value difference is defined as “SO2 reduction current,” and the SO2 concentration around the sensor is detected from an integrated value of the SO2 reduction current.