A polymerization reactor of the present invention includes a container body 1 and a jacket 2 covering the outer surface of the container body 1 and defining a passage for passing a cooling/heating medium between itself and the outer surface of the container body. The container body 1 is made of a clad metal plate including a support metal layer 11a having an inner surface at an inner side of the container body and an outer surface at an outer side of the container body, and an inner corrosion-resistant metal skin layer 11b bonded to the inner surface of the support metal layer and being smaller in thickness than the support metal layer.