In a heat exchanger that includes an expanded part of a duct, a heat-transfer tube bundle accommodating duct, and a plurality of heat-transfer tube bundles provided in the heat-transfer tube bundle accommodating duct in a flow direction of flue gas with a distance therebetween, a bare-tube-part upstream-side regulating plate and a bare-tube-part downstream-side regulating plate at an upstream side and a downstream side of a bare tube part of each of the heat-transfer tube bundles and a plurality of regulating plates in an introducing unit arranged either in the expanded part of a duct or in the heat-transfer tube bundle accommodating duct on an upstream side to the heat-transfer tube bundle are provided. A drift at the bare tube part of the heat-transfer tube bundles can be significantly reduced.