An illumination device includes a diffusion element and a light source device that emits a beam bundle including a first unit beam bundle to be incident on the diffusion element along a first direction and a second unit beam bundle to be incident on the diffusion element along a second direction. An angle formed by the first unit beam bundle with an optical axis of the beam bundle is greater than an angle formed by the second unit beam bundle with the optical axis, and the illumination device satisfies I2