PROBLEM TO BE SOLVED: To satisfy the transparency and heat resistance and further to fulfill the workability and optical isotropy which have been incompatible with heat resistance so far by using a specified polyurethane contg. an asymmetric spiro ring as the optical core and/or optical clad. SOLUTION: A polyurethane shown by the formula and contg. an asymmetric spiro ring is used as the optical core and/or optical clad. 111 the formula, R1 s are independently hydrogen or methyl, R2 s are independently a straight-chain, branched-chain or cyclic alkyl group capable of having a substituent, nitro group or halogen, (k) and (m) are independently an integer of 0 to 20, where (k+m) are never 0, (n) is an integer of 0 to 3, and R3 is an alkylene, aralkylene or arylene group. Consequently, the transparency and heat resistance are satisfied, and further the workability and optical isotropy are fulfilled which have been incompatible with heat resistance so far.