To provide an once-through exhaust heat recovery boiler capable of suppressing water hammering at a bypass water supplying mixing portion in starting a boiler.

This once-through exhaust heat recovery boiler comprises a high-pressure secondary evaporator 3, a high-pressure primary evaporator 7, a high-pressure secondary evaporator inlet communication pipe 9, a high-pressure steam separator 8, and a high-pressure secondary economizer 6, and a high-pressure bypass system 11 is disposed from an outlet side of the high-pressure secondary economizer 6 toward the high-pressure secondary evaporator inlet communication pipe 9.