Recently, studies in the United States and European countries have shown that the degree of attachment is associated with the attribution of emotions to companion animals. These studies imply that investigating the degree of attachment to companion animals is a good way for researchers to explore animal emotions and then improve animal welfare. Although a promising area of study, in Japan, no empirical studies have examined the correlation between the degree of attachment and the attribution of emotions to companion animals. In this research, we aimed to assess companion animal owners' attribution of six primary (anger, joy, sadness, disgust, fear and surprise) and four secondary (shame, jealousy, disappointment and compassion) emotions to their dogs and cats, as well as how the degree of attachment related to such attribution of emotions from a Japanese cultural perspective. The "Pet Bonding Scale" (PBS), which is used to determine the level of bonding between humans and animals, was introduced to measure respondents' degree of attachment to their companion animals. The results of a questionnaire (N = 546) distributed throughout Japan showed that respondents attributed a wide range of emotions to their animals. Companion animals' primary emotions, compared to secondary emotions, were more commonly attributed by their owners. The attribution of compassion and jealousy was reported at a high level (73.1% and 56.2%, respectively), which was surprising as compassion and jealousy are generally defined as secondary emotions. All participants were highly attached to their companion animals, and this attachment was positively associated with the attribution of emotions (9/10) to companion animals (all p < 0.05). This study is one of the first to investigate animal emotions by analyzing the bonding between companion animals and owners in Japan, and it can therefore provide knowledge to increase Japanese people's awareness of animal welfare.