An oven width measuring instrument capable of measuring the oven width continuously while being subject to no restrictions of measurement area or measuring time has: a sensor unit SU composed of an integrated combination of laser displacement sensors 16 and 17 each containing a beam emitting element and a beam receiving element in an outer package, a plurality of plate-like Peltier elements 20a-20d surrounding the outer package and arranged so as to direct their heat absorbing faces toward the outer package, an aluminum inner frame 18 for embedding gaps between the outer package and the heat absorbing faces of the Peltier elements, and cooling fin groups 21a-21d arranged on the heat radiating faces of the Peltier elements; and a housing 13 having an introduction part for introducing cooling air, a discharging part for discharging the cooling air used for cooling, and measurement windows 26 and 28 through which laser beams are passed. The sensor unit SU is accommodated in the housing.