Some researchers have been the effect of people petting a dog while measuring their brain waves using an EEG recorder. However, until now, there was little information on the effects of petting a dog. This study investigated the effects of petting a companion dog on electrical activity in the brain as evaluated by an electroencephalogram(EEG). The subject was 23 healthy male young adult. EEG electrodes were attached at the frontal, temporal, occipital, and parietal lobes according to the international 10-20 system. The subject conducted petting a retriever dog for a period of 3 minutes each before, and after petting a dog. The activity of petting a dog decreased the alpha power/beta power ratio on the frontal and parietal lobes of the brain. These decreases induced by the increased beta power. The increased beta power induced the enhancement of brain activity. It is concluded that petting a dog increases the state of activity in the brain; alpha powers/beta powers ratio was increased in the brain of subject. This is expected to be utilized to activate the brain areas for creativity of the subjects.