To provide a condition detection device for a safety device capable of carrying out early correspondence/actions even when remotely monitoring movement by accurately and safely determining detection of presence or absence of operation of a safety valve or presence or absence of reseating without relying on senses of a person.

The condition detection device is used in the safety device provided with a main steam pipe 4 carrying steam, the safety valve 6 opening when a pressure in the main stream pipe 4 reaches a predetermined value or more, and a discharge pipe 7 discharging steam flowing out from the main steam pipe 4 via the safety valve 6. It is provided with a first pressure detector 11 detecting the pressure in the main steam pipe 4, and a second pressure detector 12 detecting a pressure in the discharge pipe 7. On the basis of a difference P1-P2 between the pressure P1 detected by the first pressure detector 22 and the pressure P2 detected by the second pressure detector, presence or absence of operation of the safety valve 6 and presence or absence of stopping of steam, and an alarm responding to a determination result is generated.