A solid electrolyte of the present invention is represented by a general formula: Li.sub.aP.sub.bM.sub.cO.sub.dN.sub.e, where M is at least one element selected from the group consisting of Si, B, Ge, Al, C, Ga and S, and a, b, c, d and e respectively satisfy a=0.62 to 4.98, b=0.01 to 0.99, c=0.01 to 0.99, d=1.070 to 3.985, e=0.01 to 0.50, and b+c=1.0. The solid electrolyte hardly deteriorates in a wet atmosphere.