PURPOSE: To provide a compact measurement section protection device of an optical dimension measurement instrument of which constitution is simplified.

CONSTITUTION: A long object W to be measured conveyed in an optical path of an optical dimension-measuring instrument 1 is, for example, surrounded by a cylindrical heat-resistant glass 21 so as to prevent water sprinkled to the object W to be measured for the lubrication from being scattered to a measuring section such as lenses 13, 15 or the like. Drops of water adhered to an inner face of the glass 21 is wiped by a wiper 3. Thereby, it is possible to prevent contamination of the measurement section such as lenses.