Background: Recent years have seen an increase in the use of internet-based Cognitive Behavioral Therapy in the area of mental health. Although lower effectiveness and higher dropout rates of unguided than those of guided internetbased Cognitive Behavioral Therapy remain critical issues, not incurring ongoing human costs makes it highly advantageous. Objective: Paying due consideration to the importance of the therapeutic alliance in psychotherapy, the current research involved an evaluation of the development and efficacy, in terms of mental health, of an application with an embodied conversational agent, enabled for use as an internet-based Cognitive Behavioral Therapy preventative mental health measure. Methods: Analysis of the data from the 191 participants of the experimental group (average age 38.07 years, SD = 10.75) and the 263 participants of the control group (average age 38.05 years, SD = 13.45) using a 2-way factorial ANOVA (group × time) was performed. Results: There was a significant main effect(P=.02) and interaction for time on the variable of positive mental health(P=.02), and for the treatment group, a significant simple main effect was also found(P=.002). In addition, there was a significant main effect(P=.02) and interaction for time on the variable of negative mental health(P=.005), and for the treatment group, a significant simple main effect was also found(P=.001). Conclusions: This research can be seen to represent a certain level of evidence for the mental health application developed herein, indicating empirically that the embodied conversational agent can be useful in mental health care. Meanwhile, given the issues in the non-randomized nature of the comparison study, it is necessary to pursue higher quality evidence while continuing to further improve the application, based on the findings of the current research.