An auxiliary device for boring dowel holes in boards which are to be doweled together comprising a flat plate provided with a handle and having a centrally located guiding hole for guiding a boring tool, such as a drill, through the plate. Vertically projecting pegs are provided on each side of the flat plate to diametrically oppose the boring hole at equal distances therefrom, each peg being at the same height. One side of the flat plate is provided with a longitudinal slit in alignment directed radially toward the centrally located guiding hole. The narrow side of the board is placed at the rear of the flat plate with the pegs projecting downwardly to move the board between the pegs whereby the centrally located guiding hole is aligned at the center of the board in order that a dowel hole may be bored at the desired first location. The inside width of the longitudinal slit generally equals that of the inside width of the guiding hole with the aligned slit and guiding hole in perpendicular relationship to the contacting edge of the board to provide precise positioning of the dowel in the center of the board.