A torsional vibration measuring instrument comprising: a plurality of reflector plates (6,7,23,24,28,29) arranged with stipulated intervals therebetween along surface of a rotator of which torsional vibration is to be measured; a pulse light irradiating means (1, 2) for irradiating a repetitive pulse light beam to the plurality of reflector plates (6,7,23,24,28,29); a plurality of transmitting-receiving means (8,9,30-33) for irradiating the transmitted pulse light beam to the plurality of reflector plates (6,7,23,24,28,29) and for receiving plurality of reflected pulse light beams which have been reflected by the plurality of reflector plates (6,7,23,24,28,29); a plurality of detecting means (12, 13) for detecting the plurality of reflected pulse light beams a plurality of transmission devices (3,4,34) for transmitting the irradiated pulse light beam from the pulse light irradiating means (1, 2) to the plurality of transmitting-receiving means (8,9,30-33) and for transmitting the plurality of reflected pulse light beams from the plurality of transmitting-receiving means (8,9,30-33) to the plurality of detecting means (12, 13); and a signal processing means for processing a plurality of pulse output signals which have been outputted from the plurality of detecting means (12, 13) and for calculating torsional vibration frequency of the rotator to be measured, characterized in that the pulse light irradiating means (1, 2) is adapted to irradiate pulse light beam with variable pulse repetitive frequency and with variable pulse time width, wherein the plurality of reflector plates (6,7,23,24,28,29) includes a first reflector plate and a second reflector plate, and the pulse light irradiating means (1, 2) is adapted to irradiate a first pulse light beam and a second pulse light beam, and axial length W 1 of the first reflector plate and axial length W 2 of the second reflector plate are dimensioned to satisfy following formula: W 1 > Va > Φ, W 2 > Va > Φ wherein Va is an axial vibration amplitude of the rotator, and Φ is a beam diameter of the first and second pulse light beams on the surface of the rotator, wherein peripheral length L 1 of the first reflector plate and peripheral length L 2 of the second reflector plate are dimensioned to satisfy following formula: L 1 > > Vr > > Φ, L 2 > > Vr > > Φ wherein Vr is a vibration amplitude of the rotator in peripheral direction and perpendicular to direction of the pulse light beams on the surface of the rotator. a plurality of reflector plates (6,7,23,24,28,29) arranged with stipulated intervals therebetween along surface of a rotator of which torsional vibration is to be measured; a pulse light irradiating means (1, 2) for irradiating a repetitive pulse light beam to the plurality of reflector plates (6,7,23,24,28,29); a plurality of transmitting-receiving means (8,9,30-33) for irradiating the transmitted pulse light beam to the plurality of reflector plates (6,7,23,24,28,29) and for receiving plurality of reflected pulse light beams which have been reflected by the plurality of reflector plates (6,7,23,24,28,29); a plurality of detecting means (12, 13) for detecting the plurality of reflected pulse light beams a plurality of transmission devices (3,4,34) for transmitting the irradiated pulse light beam from the pulse light irradiating means (1, 2) to the plurality of transmitting-receiving means (8,9,30-33) and for transmitting the plurality of reflected pulse light beams from the plurality of transmitting-receiving means (8,9,30-33) to the plurality of detecting means (12, 13); and a signal processing means for processing a plurality of pulse output signals which have been outputted from the plurality of detecting means (12, 13) and for calculating torsional vibration frequency of the rotator to be measured, characterized in that the pulse light irradiating means (1, 2) is adapted to irradiate pulse light beam with variable pulse repetitive frequency and with variable pulse time width, wherein the plurality of reflector plates (6,7,23,24,28,29) includes a first reflector plate and a second reflector plate, and the pulse light irradiating means (1, 2) is adapted to irradiate a first pulse light beam and a second pulse light beam, and axial length W 1 of the first reflector plate and axial length W 2 of the second reflector plate are dimensioned to satisfy following formula: W 1 > Va > Φ, W 2 > Va > Φ wherein Va is an axial vibration amplitude of the rotator, and Φ is a beam diameter of the first and second pulse light beams on the surface of the rotator, wherein peripheral length L 1 of the first reflector plate and peripheral length L 2 of the second reflector plate are dimensioned to satisfy following formula: L 1 > > Vr > > Φ, L 2 > > Vr > > Φ wherein Vr is a vibration amplitude of the rotator in peripheral direction and perpendicular to direction of the pulse light beams on the surface of the rotator.