procedure TForm1.TreeView1DragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
begin
  //ドラッグ処理
  Accept:=False;
  if Source is TTreeView then
    if TreeView1.GetNodeAt(X,Y) <> nil then
      Accept:=True;
end;

procedure TForm1.TreeView1DragDrop(Sender, Source: TObject; X,Y: Integer);
var
  DropNode : TTreeNode;
begin
  //ドロップ処理
  DropNode := TreeView1.GetNodeAt(X, Y);
  if DropNode <> nil then
    TreeView1.Selected.MoveTo(DropNode,naAddChild);
end;