Abstract: A method of operation to effectively excavate an underdrain in the field ground having a long service life and ensuring that the underdrain will not be closed after a long period of time and a device for such operation. The method includes attaching a plurality of buckets to an endless chain, driving the endless chain to excavate a slit in the field ground, reforming inner side walls of the slit by a reformer, pressing soil from both sides of the slit later ally inwardly by a pressing member attached to the reformer to form a block to separate the slit into an upper space and a Iover space, and charging the upper space with discarded dirt.