[Unity5]how to change alpha by using shader.

I'm struggle to make fog of war watching this video.

https://www.youtube.com/watch?v=PNAvNeOTnSE

but I get stuck in some problem. Fog's alpha don't change!

Shader "Custom/FogOfWarMasking" {

 Properties {
     _Color("Main Color", Color) = (1,1,1,1)
     _MainTex ("Base (RGB)", 2D) = "white" {}
 }
 SubShader {
     Tags { "Queue"="Transparent" "RenderType"="Transparent" "LightMode"="ForwardBase"}
     Blend SrcAlpha OneMinusSrcAlpha
     Lighting off
     LOD 200
     
     CGPROGRAM
     #pragma surface surf NoLighting noambient

     fixed4 LightingNoLighting(SurfaceOutput s, fixed3 lightDir, float aten)
     {
         fixed4 color;
         color.rgb = s.Albedo;
         color.a = s.Alpha;
         
         return color;
     }
     fixed4 _Color;
     sampler2D _MainTex;

     struct Input {
         float2 uv_MainTex;
     };

     void surf (Input IN, inout SurfaceOutput o) {
         half4 baseColor = tex2D (_MainTex, IN.uv_MainTex);
         
         o.Albedo = _Color.rgb * baseColor.b;
         o.Alpha = _Color.a - baseColor.g; //green - color of aperture mask
     }
     ENDCG
 } 

 Fallback "Diffuse"

 }

Everything is ok except transparent...

What was worse, set o.Alpha specific value like 0.0f, 0.5f, It doesn't work either. :(

This tutorial worked well in Unity4.x, Unity5.x not. but I don't know why alpha was not changed.

To sum up, how to change alpha to make transparent shader in Unity5?