To provide a device for detecting an operation state of valves, the device that facilitates attaching a sensor for operating state detection to a part to be detected.

An operation state detecting device 2 includes: a sensor 3 for operating state detection, which is abutted to an entrance 1 of a steam trap which is a part to be detected; a holding member 4 that is roughly a square shape in a cross section having an upper wall 4a, both sidewalls 4b and a lower wall 4f, has the sensor 3 for the operating state detection connected to the upper wall 4a and the lower wall 4f, is also provided with a pair of insertion holes 4d, and is provided with a notched part 4e in communication with one insertion hole 4d and for inserting one leg part 6a of a U bolt 6 on the upper wall 4a or one sidewall 4b or the lower wall 4f; the U bolt 6 whose both leg parts 6a are inserted to the pair of insertion holes 4d on the upper wall 4a and the lower wall 4f of the holding member 4; and a pair of nuts 7 attached to both leg parts of the U bolt 6.