The purpose of this study was to develop a Scale for Shared Mealtime Quality (SSMQ), and to investigate the relationships between Shared Mealtime Quality (SMQ) and life stages or mental health, after first defining SMQ. As a result of our preliminary study, SMQ was defined as “sharing enjoyment in the presence of others at mealtime”, and 27 items were collected for the scale. Factor analysis in our main study showed that SSMQ consisted of three factors: “family” (7 items), “friend” (7 items), and “childhood eating habits.” Hierarchical confirmatory factor analysis indicated that SSMQ possessed three factors constructing SMQ, and one general factor of comprehensive SMQ. For the friend and family factors, one-way analysis of variance revealed significant differences in three individual life stages (adulthood, late middle age, and old age). Multiple comparisons revealed that old age showed a higher friend factor score than that of adulthood, a higher family factor score than those of late middle age and adulthood, and late middle age showed a higher family score than that of adulthood. Additionally, correlation analysis indicated that friend and family factors were negatively related to mental health.