A method for manufacturing a hollow plastic product. The method includes the steps of forming a reinforce member in a desired shape having a defined welding surface formed with a first hole by injection molding; forming a hollow main body by blow molding using said reinforce member as an insert, thereby having said reinforce member integrally formed with said hollow main body and having a portion of said hollow main body extended into said first hole in the shape of a mesa when blow-molded; thermally welding a top end surface of a connector pipe to said welding surface and also to a top surface of said mesa shaped portion; and forming a second hole in said top surface of said mesa shaped portion to be aligned with said second hole, thereby forming a fluidic communication passage between an interior of said hollow main body and an interior of said connector pipe.