PROBLEM TO BE SOLVED: To provide an optical characteristic measurement instrument which is superior in dust-proof and splash-proof performance and is easy to clean and can be adapted to even measurement using no sample cups and measurement without rotation of a sample.SOLUTION: The optical characteristic measurement instrument includes a measurement base 17 fixed to an opening 16b of an instrument casing 16, an electric motor 32 which has a revolving shaft 32a made to face the inside of a positioning boss 17b of the measurement base 17, and a sample rotating unit 18 placed on the measurement base 17. The sample rotating unit 18 comprises an upper cover 34, a sample stage 22, a gear (driving shaft) 36, and a base plate 37. When the sample rotating unit 18 is placed on the measurement base 17, a positioning hole 37a of the base plate 37 is fitted to the positioning boss 17b of the measurement base 17, whereby the sample rotating unit 18 is positioned on the measurement base 17 and the gear 36 is connected to th
