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, reduction of the size and weight, and prevention of temporal changes 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. A transparent substrate 50 is bonded via a seal 51, and temporal changes of a light emitting element 10 and electrodes which are sealed therein are suppressed.