A signal measuring device measuring process signals in a signal measurement target section,the signal measurement target section including a plurality of detection units (101, 102, 103, 1M),each detection unit (101, 102, 103, 1M) detecting a continuously varying process signal, the signal measuring device comprising: - one input circuit unit for receiving the process signal from each detection unit (101, 102, 103, 1M); - a signal switching unit for performing open/close switching between each detection unit (101, 102, 103, 1M) and the input circuit unit; - a signal processing unit connected to the input circuit unit and for processing the process signal; and - a switching command unit for outputting, to the signal switching unit, a switching command signal that corresponds to an open/close time set such that one of the detection units (101, 102, 103, 1M) is connected to the input circuit unit, the switching command unit for outputting, to the signal processing unit, a measurement command signal (t) that corresponds to a stable measurement time which is set for each detection unit (101, 102, 103, 1M) and after which the process signal inputted from the detection unit (101, 102, 103, 1M) to the input circuit unit is stably measurable, wherein the signal switching unit includes a fourth input terminal (17A, 17B, 17C, 17N), a fourth output terminal (18A, 18B, 18C, 18N), a power supply unit (19A, 19B, 19C, 19N), and a load resistance, for the detection unit (105, 106, 107, 1N) that needs a power supply, the detection unit (105, 106, 107, 1N) that needs a power supply, the power supply unit (19A, 19B, 19C, 19N), and the load resistance (20A, 20B, 20C, 20N) are connected in series, the load resistance (20A, 20B, 20C, 20N) and the input circuit unit are connected in parallel, and are connected via the fourth input terminal (17A, 17B, 17C, 17N) and the fourth output terminal (18A, 18B, 18C, 18N), and the switching command unit outputs the switching command signal to the fourth input terminal (17A, 17B, 17C, 17N) and the fourth output terminal (18A, 18B, 18C, 18N), each of them being composed of a switching mechanism which performs opening/closing. - one input circuit unit for receiving the process signal from each detection unit (101, 102, 103, 1M); - a signal switching unit for performing open/close switching between each detection unit (101, 102, 103, 1M) and the input circuit unit; - a signal processing unit connected to the input circuit unit and for processing the process signal; and - a switching command unit for outputting, to the signal switching unit, a switching command signal that corresponds to an open/close time set such that one of the detection units (101, 102, 103, 1M) is connected to the input circuit unit, the switching command unit for outputting, to the signal processing unit, a measurement command signal (t) that corresponds to a stable measurement time which is set for each detection unit (101, 102, 103, 1M) and after which the process signal inputted from the detection unit (101, 102, 103, 1M) to the input circuit unit is stably measurable, wherein the signal switching unit includes a fourth input terminal (17A, 17B, 17C, 17N), a fourth output terminal (18A, 18B, 18C, 18N), a power supply unit (19A, 19B, 19C, 19N), and a load resistance, for the detection unit (105, 106, 107, 1N) that needs a power supply,