A gas temperature control apparatus for a DeNOx boiler in an engine cogeneration plant is installed to maintain a reaction temperature of denitrification catalyst used to remove nitrogen oxide generated from an internal combustion engine in the engine cogeneration plant. The apparatus includes a connection passageway between a high-temperature gas part of a combustion tank of the boiler and an intermediate-temperature gas part formed after a first heat exchange, and a flow control valve mounted in the passageway to thereby control a volume of bypass combustion gas. The apparatus can easily raise temperature of combustion gas even though temperature of the inside water of the boiler does not reach a normal operation state since the flow control valve mounted in the connection passageway between the high-temperature gas part and the intermediate-temperature gas part. thereby greatly reducing a time period to reach an activation temperature of the denitrification catalyst.