A III-V-group compound semiconductor device includes a substrate, a channel layer provided over the substrate, a barrier layer provided on the channel layer so as to form a hetero-interface, a plurality of electrodes provided on the barrier layer, an insulator layer provided to cover an entire upper surface of the barrier layer except for at least partial regions of the electrodes, and a hydrogen-absorbing layer stacked on the insulator layer or an integrated layer in which an hydrogen-absorbing layer is integrated with the insulator layer.