Remove["Global`*"];
Timing[
frq = 1; 
shuki = 
          1/frq; 
r[x_, y_, x0_, y0_] := 
          Sqrt[(x - x0)^2  +  (y - y0)^2]; 
warur[
            x_, y_, x0_, y0_] := 
          If[Sqrt[(x - x0)^2  +  (y - y0)^2] == 0, 
            1, (1 - Exp[-Sqrt[(x - x0)^2  +  (y - y0)^2]])/
              Sqrt[(x - x0)^2  +  (y - 
                        y0)^2]]; 

velo = 4; 
hachou = 
          velo/frq; 

nami[t_, x_, y_, 
            x0_, y0_] := 
          If[(velo*(t - shuki) < 
                  r[x, y, x0, y0])  &&  (r[x, y, x0, y0] < velo*t), 
            Sin[-2*
                  Pi*(r[x, y, x0, y0] - velo*(t - shuki))/hachou]*
              warur[x, y, x0, y0], 
            0]; 

t = 
          3; 
x0 = 0; 
y0 = 
          0; 

Plot3D[
          nami[t, x, y, x0, y0], {x, -20, 20}, {y, -20, 20}, 
          PlotRange  ->  {-0.5, 0.5}, 
          PlotPoints  -> 80];
]