An illumination device has a plurality of sealing structures each including a light-emitting diode, sealed by a high refractive index transparent material member which is further sealed by a low refractive index transparent material member. To increase a divergence angle of light radiation, the portion of the high refractive index transparent material member which covers an upper surface of the light-emitting diode is structured so as to satisfy a relation H>(L/2)/tan {sin−1(n1/n2)}, where H represents a thickness of the high refractive index transparent material member measured at the central portion of the upper surface of the light-emitting diode, L represents the length of one side of the upper surface of the light-emitting diode, n1 represents the refractive index of the high refractive index transparent material member and n2 represents the refractive index of the low refractive index transparent material member.