A method for measuring a protrusion length of a protrusion portion formed at both width end sides of a long sheet having a plurality of parallelly arranged cords embedded therein, the method comprising: generating waveform data by imaging a cross-sectional profile in a width direction of the sheet including the protrusion portion by an imaging means (30a, 30b); detecting maximum height positions (Pa) of crests and minimum height positions (Pc) of troughs of the waveform data; and, for each width end of the sheet, the method comprising: setting, based on the waveform data, a maximum height position (Pa) of a crest closest to the width end of the sheet as a maximum height position (Pea) of an end crest; setting a position corresponding to an edge of the protrusion portion of the sheet, which is located on more width end side than the maximum height position (Pea) of the end crest, as a minimum height position (Pec) of a terminal trough; detecting a distance (Xm) in a width direction of the sheet between the maximum height position (Pea) of the end crest and the minimum height position (Pec) of the terminal trough; measuring the protrusion length of the protrusion portion based on the detected distance (Xm); calculating a reference position being a reference point a predetermined distance (Ym, Yn) apart in the width direction from the maximum height position (Pea) of the end crest; and moving a cutter in a direction to decrease the protrusion length, when the protrusion length exceeds the predetermined distance (Ym, Yn) of the reference position or moving the cutter in a direction to increase the protrusion length, when the protrusion length does not reach the predetermined distance (Ym, Yn) of the reference position. generating waveform data by imaging a cross-sectional profile in a width direction of the sheet including the protrusion portion by an imaging means (30a, 30b); detecting maximum height positions (Pa) of crests and minimum height positions (Pc) of troughs of the waveform data; and, for each width end of the sheet, the method comprising: setting, based on the waveform data, a maximum height position (Pa) of a crest closest to the width end of the sheet as a maximum height position (Pea) of an end crest; setting a position corresponding to an edge of the protrusion portion of the sheet, which is located on more width end side than the maximum height position (Pea) of the end crest, as a minimum height position (Pec) of a terminal trough; detecting a distance (Xm) in a width direction of the sheet between the maximum height position (Pea) of the end crest and the minimum height position (Pec) of the terminal trough; measuring the protrusion length of the protrusion portion based on the detected distance (Xm); calculating a reference position being a reference point a predetermined distance (Ym, Yn) apart in the width direction from the maximum height position (Pea) of the end crest; and moving a cutter in a direction to decrease the protrusion length, when the protrusion length exceeds the predetermined distance (Ym, Yn) of the reference position or moving the cutter in a direction to increase the protrusion length, when the protrusion length does not reach the predetermined distance (Ym, Yn) of the reference position. setting, based on the waveform data, a maximum height position (Pa) of a crest closest to the width end of the sheet as a maximum height position (Pea) of an end crest; setting a position corresponding to an edge of the protrusion portion of the sheet, which is located on more width end side than the maximum height position (Pea) of the end crest, as a minimum height position (Pec) of a terminal trough; detecting a distance (Xm) in a width direction of the sheet between the maximum height position (Pea) of the end crest and the minimum height position (Pec) of the terminal trough; measuring the protrusion length of the protrusion portion based on the detected distance (Xm); calculating a reference position being a reference point a predetermined distance (Ym, Yn) apart in the width direction from the maximum height position (Pea) of the end crest; and moving a cutter in a direction to decrease the protrusion length, when the protrusion length exceeds the predetermined distance (Ym, Yn) of the reference position or moving the cutter in a direction to increase the protrusion length, when the protrusion length does not reach the predetermined distance (Ym, Yn) of the reference position.