An optical time domain reflectometer (OTDR) launches an optical pulse emitted from a pulse light source to one end side of an optical fiber, receives light returning to the one end side with an optical receiver, inputs an output signal of the optical receiver to an A/D converting unit so as to convert the signal to an N bit (N is the plural number) digital signal, implements a process for the N bit digital signal, and measures the transmission characteristics of the optical fiber. The A/D converting unit has the number of bits that is greater than the N bits by the number of A bits, wherein A is an integer greater than "0". The A/D converting unit converts an input signal to an N+A bit digital signal, and compresses the N+A bit digital signal to N bit data.