To enable a continuous operation for a long term by preventing a heat exchanger for cooling reaction gas of a fluidized bed reactor from being clogged.

In the multi-tubular cylindrical heat exchanger used for cooling the reaction gas of fluidized bed reaction, a straightening device 3 for gas flow is provided in an inlet part for the reaction gas of the heat exchanger. As a specific shape of a straightening section, one of a structure having the inlet part of the heat transfer pipe chamfered at its inner face side, a reducer structure or a bell-mouthed tube structure is employed for the heat exchanger for fluidized bed reaction 2.