Tail-wagging is thought to be a behavioral response reflecting an emotional state of an individual animal, and to play an important role as an honest signal in conspecific interactions. Domestic dogs Canis familiaris are shown to wag their tail from side-to-side when being excited or when approaching to a familiar person in a friendly manner. However, there is little empirical evidence about the social contexts that trigger dog’s tail-wagging and its social function. To test what types of dog-human interactions elicit the most intense tail wag in dogs, Study 1 examined dog’s tail-wagging responses during a variety of social interactions including feeding by, playing and greeting with an owner. I found that dogs wagged their tail for longer periods of time during the play as well as the reunion with an owner than during the feed by an owner. Additionally, amplitude of tail-wagging was larger during the reunion with an owner than during the play with a ball and in a tug-of-war. The results indicate that dog’s tail-wagging is not a mere reflex response, but is modulated by qualitative differences in social contexts. To test whether dog’s tail-wagging is contagious or not, Study 2 examined dog’s behavioral responses when looking at a model dog wagging its tail. I found that dogs reacted differently according to the visual stimuli, with dogs wagging their tail more intense and getting closer to the stimulus when looking at a model dog wagging its tail than when looking at those not wagging it. The results suggest that a movement of tail-wagging holds dog’s attention and the dogs coordinate their behavior contagiously with the action of a conspecific model. Social modulation of dog’s tail-wagging may have a significance of maintaining and reinforcing a social relationship both of cross-specific and conspecific interaction. The possible contagion of tail-wagging between dogs may involve transmission of emotional state, which is related to a primitive form of empathy.