An illuminating device (10) is provided with a casing configured from an outer cover (11) having a U shape, a transparent cover (12) provided so as to cover the opening of the U shape, a first side cover (13) provided on the left-hand side, and a second side cover (14) provided on the right-hand side. Further, the illuminating device (10) is provided with a light-emitting element package (21) mounted with a semiconductor light-emitting element in the casing, a circuit board (22) mounted with the light-emitting element package (21), a heat-dissipating base (24) to which the circuit board (22) is secured and which a coolant conduit (25) is provided in close contact with, a filmy insulating heat-dissipating member (23) for electrically insulating the circuit board (22) from the heat-dissipating base (24), and a reflector (26) for reflecting light. The inside of the casing is sealed. Therefore, the problems due to dew condensation and moisture absorption while the semiconductor light-emitting element is efficiently cooled by means of a coolant, such as water are solved, and an illuminating device for plant cultivation using the semiconductor light-emitting element, which has a long life and can maintain high efficiency, as the light source and a plant cultivation system using the same are provided.
