This study analyzes the relationship between work engagement (WE) and job performance based on data from 3,296 individuals obtained in an internet survey. First, the analysis was conducted with WE as an explanatory variable, as well as with job performance, which was measured as the difference in personal and workmate evaluations, as an objective variable, and yet no clear relationship was found. Job performance was then used as an explanatory variable and WE as an objective variable, and that analysis discovered a clear inverse U relationship between the two. In other words, WE is low when job performance is low or high, and high when job performance is in the middle range. Also, when an analysis was done with the prediction that sum of, rather than differences in, personal and workmate assessments will have a strong relationship with WE, it was found that there was a clear, positive linear relationship between the two. In other words, the more a person highly evaluates not only oneself but also one’s workmates, the higher the WE. Existing studies tend to believe that WE heightens job performance, yet this study indicates that the converse is perhaps true in this cause and effect relationship. It also suggests that WE increases as one builds healthy relationships with workmates, evaluates them highly, and works hard and competes in a friendly manner with them.