To enable recovery of incineration ash with less caesium ash from exhaust gas in a sewage sludge incinerator and dispense with addition of chemicals to the incineration ash that becomes high cost and caesium removal treatment from drainage.

Exhaust gas of a sewage sludge incinerator 1 for incinerating sewage sludge in an oxidizing atmosphere is introduced into a high-temperature cyclone 3 at a high-temperature atmosphere of ≥678°C that volatilizes caesium, and most of the incineration ash is separated and recovered as high temperature dust ash 4 with a low caesium content. A part of the incineration ash is made to pass through the high-temperature cyclone 3 together with the exhaust gas, the caesium volatilized in the exhaust gas is re-caught with the incineration ash in a process of temperature falling. This incineration ash is separated and recovered as low temperature dust ash 7 at a low temperature atmosphere of ≤250°C by a filter type dust separator 6, such as a bag filter or a ceramic filter.