In a light irradiation device in which a light emitting element is attached to a printed circuit board, the heat radiation properties are enhanced, and improvement of the light emitting efficiency and reduction of the size and weight are realized. A Cu pattern covered with Ni is formed on a metal substrate 11. Light emitting elements 11 are mounted on the pattern in the form of a series circuit. Metal substrates in each of which the series connection is formed are connected to one another in parallel. Since Ni has excellent corrosion resistance and a high reflection efficiency, the surfaces of the substrates themselves can be used as reflective plates. A lens 37 is formed for each of the light emitting elements, whereby the emission efficiency can be further improved. When a circuit is configured in one region of a hybrid integrated circuit substrate, a position recognition mark 53 and a flow stop 57 are disposed in the open region. Also the region is covered with Ni so as to function as reflecting means.