PURPOSE: To obtain a detector which can measure the rate of progression in taking-in of data even if normal data are continuous or intermittent by providing a two-color light emitting diode and displaying the rate of said progression from the change in the colors thereof.

CONSTITUTION: A two-color light emitting diode 63 consisting of a red light emitting element 61 and a green light emitting element 62 is provided, and a color display circuit is constituted of said diode and a digital-to-analog converter 64 which converts the data taking-in times obtd. from a data processing circuit 42 to analog quantity. The element 61 is connected to the output terminal of the converter 64 and the element 62 to the output terminal of a level decision circuit 24. An internal path circuit 30 consisting of an optical fiber 31 and a shutter 32 and a circuit 40 consisting of a phase detecting circuit 41 and a data processing circuit 42 including a counter are provided.