A method of manufacturing a light emitting element mounting base member includes: arranging a plurality of core members each including an electrical conductor core and a light-reflecting insulating member provided on a surface of the electrical conductor core; cutting the arranged core members to form a base member preparatory body including at least one cut surface on which at least one of the electrical conductor cores and the insulating members are exposed; and insert molding by placing the base member preparatory body in a set of mold, and injecting a light blocking resin composition into the set of mold such that at least one of the electrical conductor cores or at least one metal film formed on at least one of the electrical conductor cores are exposed on at least one outer surface of the light emitting element mounting base member.