To provide a lens with which different light distribution characteristics can be obtained easily, to provide a lens mounting substrate to be mounted with the lens, and to provide an illumination device equipped with the lens and the lens mounting substrate.

The lens 1 is equipped with a lens part 11 to demarcate the light distribution of light incident from the light source 3, and a leg body 12 that is extended from the lens part 11 and engaged with the lens mounting substrate 2 to support the lens part 11. The leg body 12 is equipped with a level difference part 13 made to cope with a shape of an engaging part 21 that the lens mounting substrate 2 has. An engaging position of the leg body 12 against the lens mounting substrate 2 is changed, the engaging position of the leg body 12 against the lens mounting substrate 2 is changed in an optical axis Lax direction, and the position of the lens part 11 (lens 1) against the light source 3 is made different in the optical axis Lax direction, so that a different light distribution characteristic Lc can be obtained easily by the same lens 1.