To provide a side seal bite detection device capable of correctly detecting bite by eliminating fluctuation errors in the result of distance measurement.

Eddy current displacement sensors 200 and 201 and a proximity sensor 202 are installed facing side seals 72. When the two eddy current displacement sensors measure the distance to each side seal, the measurement is performed only during the time while an optimum facing condition is maintained, and only the correct measurement data can be selected. Fitting errors of a side seal bar, errors caused by a thermal expansion of the side seal bar itself or the like are eliminated by comparing the distance measurement data for detecting any bite with the result of the once previous measurement, and correct bite can be detected thereby.