To provide a projection type display device in which a large increase in the cost is prevented, moire fringes are made unrecognizable and the picture quality is improved.

The spatial frequency of moire fringes is determined by the differences between the spatial frequency of the periodic structure of the projected picture and its higher harmonics and the spatial frequency of the periodic structure of the screen. A moire wavelength L is given by 1/L=1/S-n/Q where S is the periodic structure of a screen, (n) is the order of higher harmonics and Q is the picture element pitch of the projected picture. If the above equation is normalized by Q, it becomes 1/(L/Q)=1(S/Q)-n. This equation shows the relationship between L/Q and S/Q. If S/Q is given, the moire wavelength of each order (n) is obtained. If (m) is a positive integer, the longest moire wavelength becomes a minimum at the cross point of a curve n=m and a curve n=m+1. In the above equations, let n=m and n=m+1, them the longest moire wavelength becomes the minimum for Q/S=n+1/2.