A waste heat recovery apparatus comprising a high-­temperature unit (1) including a high-temperature casing having an inlet through which an exhaust gas is intro­duced into the unit and an outlet through which the exhaust gas is discharged from the unit, a set of heat exchanger tubes (6) arranged in the high-temperature casing, and a dry-type cleaner (7) for removing dust on the heat exchanger tubes, a low-temperature unit (2) in­cluding a low-temperature casing having an inlet through which the exhaust gas is introduced into the unit and an outlet through which the exhaust gas is discharged from the unit, a set of heat exchanger tubes (6) arranged in the low-temperature casing, and a flush-type cleaner (8) for removing dust on the heat exchanger tubes by spray­ing flushing water on the tubes, an intermediate duct (3) connecting the outlet of the high-temperature unit (1) and the inlet of the low-temperature unit (2) so that the exhaust gas from the high-temperature unit is passed into the low-temperature unit through the inter­mediate duct, an exhaust duct (5) connected to the out­let of the low-temperature unit so that the exhaust gas is discharged from the low-temperature unit through the exhaust duct, a by-pass duct (10) connecting the inter­mediate duct and the exhaust duct, first damper (9) for controlling the flow of the exhaust gas from the inter­mediate duct to the low-temperature casing, and second damper (11) for controlling the flow of the exhaust gas from the intermediate duct to the by-pass duct.