A half mirror includes a silver layer and an anti-aggregation layer in contact with the silver layer. The anti-aggregation layer may be composed of ITO or IGO. Alternatively, the anti-aggregation layer may be composed of an organic molecular film having a thiol group. Alternatively, the anti-aggregation layer may be composed of an alloy including silver in an amount of 97% or more and an element X, in which the element X is any one of Au, Mg, Zn, Cu, Al, Si, Pd, Sn, Pt, Ti, and Cr.