An apparatus for winding, in the form of a quadrupole, an optical fiber used for forming a sensor coil of a fiber optic gyroscope. The gas bubbles in the slurry are in a churn-turbulent flow regime. The drainage catheter assembly is operated between a pre-assembled condition, a first step condition and a second step condition. A length RSm in a lateral direction of concaves and convexes on the substrate surface is 55 \u03bcm or more and not more than 500 \u03bcm. The invention also relates to the base material itself.