The inventive electronic component mounting apparatus for mounting an electronic component on a substrate includes a substrate positioning unit for individually positioning a single large-sized substrate or two small-sized substrates carried into a mounting conveyor in respective mounting work positions and a substrate underside support part arranged below the mounting conveyor and including a first underside support part and a second underside support part. It is thus possible to position a single substrate in a mounting work position in case a large-sized substrate is handled and to individually position a plurality of substrates in a plurality of mounting work positions in case a small-sized substrate is handled. This ensures flexible component mounting work on plural types of substrates by a compact facility.