To provide a device and method for detecting the tension of a strip member accurately.

A tension-detecting device 10 for detecting the tension of a strip member 12 when the strip member 12 travels is provided with a plurality of guide members 14, 16, and 18 for guiding the strip member 12 while being lifted by jetting out compression gas, a pressure adjustment part 22 for constantly maintaining the pressure of the compression gas being supplied to the plurality of guide members 14, 16, and 18, and a displacement detection part 24 for converting the displacement of the guide member 16 being generated according to the tension of the strip member 12 to an electrical signal.