It is an object of the present invention to provide a production method of a dehydration reaction product by which the occurrence of troubles in the production step or the degradation in performance characteristics or quality of various chemical products can be suppressed to a satisfactory extent. A production method of a dehydration reaction product

• which comprises a dehydration reaction step of subjecting a reaction solution containing a polymerizable compound to the dehydration reaction,
• said dehydration reaction step comprising using a dehydration reaction apparatus,
• said dehydration reaction apparatus comprising a reaction vessel, a condenser and a connecting pipe joining said reaction vessel with said condenser and
• satisfying the requirement:

  0.05(B³/A)35

  where A is a capacity (m³) of said reaction vessel and B is a total length (m) of said connecting pipe on the horizontal basis.