To provide a reaction catalyst which is capable of easily removing a solid acid catalyst from the ester product to a degree so that there is no trouble with the stability of the product after the completion of the reaction.

In the esterification or urethanation reaction catalyst in which the powdery solid acid catalyst is immobilized on the supporting structure via a binder, the powdery solid acid catalyst is obtained by having a molybdic oxide carried by zirconia as a carrier and the Hammett acidity function (H0) has a value between through -3 to -9.