To obtain the subject monomers capable of effectively preventing discoloration of basic monomers and further preventing the discoloration at the time of handling such as storage or transportation by adding a specific additive.

The differences ΔL*, Δa* and Δb* of an L*a*b* color system of the basic monomers according to a forced discoloration test method are respectively ≤3, ≤3 and ≤8. ΔL*=value of L* after the test-value of L* before the test, Δa*=value of a* after the test-value of a* before the test and Δb*=value of b* after the test-value of b* before the test (ΔL*, Δa* and Δb* are each an absolute value). The objective basic monomers are preferably represented by the formula (R1 is H or methyl; R2 is a 2-4C alkylene; R3 and R4 are each a 1-8C alkyl; X is O or NH).