To serve a sensor mounting space with a share for sensor to facilitate sensor mounting.

In a seat state detecting device which detects if a long stencil base paper 1 is set or not and detects an end mark part of the long stencil base paper 1, a light detecting sensor 20 adapted to output a level output capable of judging three states among a state absent of the long stencil base paper 1, a state present of the same, and a passing state of the end mark part.