A light emitting element assembly includes: a light emitting element (21); a light emitting element drive unit (30); a first joining member (41) connected to an electrode provided in the light emitting element (21); and a second joining member (42) provided on the light emitting element drive unit (30). In the light emitting element assembly, one of the first joining member (41) and the second joining member (42) includes an alloy material, the other one of the first joining member (41) and the second joining member (42) includes a metallic material, the joining member (the second joining member (42)) including the alloy material includes a first portion (43) and a second portion (44), the joining member (the first joining member (41)) including the metallic material and the first portion (43) are joined to each other, and the second portion (44) is provided on the light emitting element drive unit (30) between the light emitting element (21) and the light emitting element drive unit (30), and is in contact with the light emitting element (21).