A light diffusion member constituting a head-up display device includes multiple micro-lenses and has a curved base surface. The edge angle is larger than the base surface angle change amount. The edge angle is an angle formed by one of tangent lines of two micro-lenses and the other of the tangent lines passing through a boundary between two adjacent micro-lenses on a cross section orthogonal to the base surface. A reference line has a constant direction on the cross section, and a normal line passes at an arbitrary position on the base surface. The maximum value is θmax and the minimum value is θmin among angles between the normal line and the reference line. The base surface angle change amount is the absolute value of the difference between θmax and θmin.