The spread of social media has caused the problem of opinion polarization as an unexpected side effect. Therefore, it is important to elucidate the mechanism of opinion formation through communications on social media. In this paper, we clarify the relationship between the degree of similarity among topics and opinion formation in multi-dimensional opinion formation that deals with multiple topics. For this objective, we use the multi-dimensional opinion formation model that we have proposed in the past. This model has the advantage of being able to avoid the "curse of dimensionality" and of being able to handle relationships among topics. We focus on how much the formation of an opinion on a particular topic influences the opinion held on other topics. Numerical experiments show that the higher the similarity between topics, the stronger the influence.