Provided is an illumination device capable of preventing a reflection sheet in the periphery of a light-emitting element from being raised while minimizing the instability in the fixture of a diffusion lens. The illumination device comprises a light-emitting element, a mounting substrate, a diffusion lens, and a reflection sheet having a through-hole. The diffusing lens includes a lens portion for diffusing light, and a fixed portion to be fixed to the mounting substrate. An engaging portion that sandwiches the reflection sheet between the engaging portion and the mounting substrate is formed in the fixed portion.