A heat exchanger for an easily polymerizing substance-containing gas provided with a shell possessed of a heat-exchanging gas inlet and a heat-exchanging gas outlet and a heat-exchanging part adapted to circulate fluid introduced from outside the shell between the gas inlet and said gas outlet, which heat exchanger is characterized by being provided with a gas distributing plate between the gas inlet and the heat-exchanging part. The heat exchanger is characterized by the gas distributing plate having a cross-sectional area in the range of 1.0-10.0 times the cross-section of the gas inlet. When an easily polymerizing substance-containing gas contacts a structure, the gas is condensed on the contact surface of the structure and suffered to generate a polymer. According to the heat exchanger of this invention, by uniformly distributing a gas in the heat-exchanging part, it is made possible to attain uniform distribution of heat, depress the possible condensation of the gas, and prevent the easily polymerizing substance from succumbing to polymerization.