An air supply device of a gas engine having a turbocharger includes a first intake passage configured to guide the mixture of outdoor air and unpurified gas into the turbocharger, a second intake passage configured to guide indoor air into the turbocharger, a filter configured to remove a solid impurity in the unpurified gas which is disposed in the first intake passage, a first damper capable of opening and closing the first intake passage disposed on the downstream side of the filter in the first intake passage, a heating unit for heating the indoor air disposed in the second intake passage, a second damper capable of opening and closing the second intake passage disposed on the downstream side of the heating unit in the second intake passage, and a damper control device configured to control the opening degree of each of the first and second dampers.