Objective:The present study aimed to compare the social function between mild cognitive impairment (MCI), mild Alzheimer’s disease (AD) dementia, and mild dementia with Lewy bodies (DLB) using the Japanese version of Social Functioning in Dementia scale (SF-DEM-J).Methods:We interviewed 103 patients and family caregivers from June 2020 to March 2021: 54 patients with MCI, 34 with mild AD dementia, and 15 with mild DLB. We compared the caregiver-rated SF-DEM-J, Clinical Dementia Rating (CDR), MMSE, age, length of education, Geriatric Depression Scale (GDS), the University of California, Los Angeles Loneliness Scale (UCLA-LS), Neuropsychiatric Inventory (NPI), and informant version of the Apathy Evaluation Scale (AES) between MCI, mild AD dementia, and mild DLB groups using Kruskal-Wallis test with Dunn-Bonferroni correction for post-hoc analyses. We compared sex, living situation, and caregiver demographics between three groups using chi-square test. We performed correlation analysis between the score of each psychological test and the scores of SF-DEM-J within group using Spearman’s rank correlation coefficient.Results:For SF-DEM-J, the score of section 2 (communicating with others) was significantly worse in mild AD dementia than in mild DLB. The scores of section 1 (spending with others) and section 3 (sensitivity to others) and the total score did not significantly differ between three groups. The score of section 1 was significantly associated with MMSE in MCI, with anxiety and disinhibition of NPI, and AES in mild AD dementia, and with GDS in mild DLB. The score of section 2 was significantly associated with AES in MCI and mild AD dementia, with UCLA-LS in MCI, and with the length of education in mild DLB. The score of section 3 was significantly associated with agitation and irritability of NPI in MCI and mild AD dementia. The total score was associated with UCLA-LS and AES in MCI, and with AES in mild AD dementia.Conclusion:Factors affecting social functioning differed between MCI, mild AD dementia, and mild DLB. Apathy, agitation and irritability affected social functioning in MCI and mild AD dementia while depressive mood affected social functioning in mild DLB.