An exhaust-gas control device for a gas engine which mixes fuel gas and air and supplies mixed gas to a combustion chamber, comprising an ignition-timing control unit configured to control an ignition timing of the mixed gas supplied into the combustion chamber, and an air-fuel rate control unit configured to control an air-fuel rate in the combustion chamber, characterized in that the exhaust-gas control device comprises: a basic operation mode unit configured to control the ignition-timing control unit and the air-fuel rate control unit, having a first target ignition timing and a first target air-fuel rate as target values which are set as an ignition timing and an air-fuel rate at which a fuel consumption rate is the best on the basis of an engine speed and an engine load; a NOx reduction demand determination unit configured to determine that there is a necessity to restrict an amount of NOx emission temporarily if an ambient temperature in which the gas engine is installed is a predetermined value or more and a humidity is a predetermined value or less; a low NOx operation mode unit configured to control the ignition-timing control unit and the air-fuel rate control unit, having a second target ignition timing retarded from the first target ignition timing and a second target air-fuel rate increased from the first target air-fuel rate as target values; and a switching unit to operation mode configured to switch an operation mode to the low NOx operation mode when it is determined by the NOx reduction demand determination unit that there is a necessity to restrict the amount of NOx emission, and configured to, when it is determined by the NOx reduction demand determination unit that there is no necessity to restrict the amount of NOx emission, maintain the operation mode if the operation mode is the basic operation mode, and switch the operation to return to the basic operation if the operation mode is the low NOx operation mode. a basic operation mode unit configured to control the ignition-timing control unit and the air-fuel rate control unit, having a first target ignition timing and a first target air-fuel rate as target values which are set as an ignition timing and an air-fuel rate at which a fuel consumption rate is the best on the basis of an engine speed and an engine load; a NOx reduction demand determination unit configured to determine that there is a necessity to restrict an amount of NOx emission temporarily if an ambient temperature in which the gas engine is installed is a predetermined value or more and a humidity is a predetermined value or less; a low NOx operation mode unit configured to control the ignition-timing control unit and the air-fuel rate control unit, having a second target ignition timing retarded from the first target ignition timing and a second target air-fuel rate increased from the first target air-fuel rate as target values; and a switching unit to operation mode configured to switch an operation mode to the low NOx operation mode when it is determined by the NOx reduction demand determination unit that there is a necessity to restrict the amount of NOx emission, and configured to, when it is determined by the NOx reduction demand determination unit that there is no necessity to restrict the amount of NOx emission, maintain the operation mode if the operation mode is the basic operation mode, and switch the operation to return to the basic operation if the operation mode is the low NOx operation mode.