thereby an ingot of a gan bulk crystal is obtained by slicing the gan bulk crystal ingot thus obtained it is possible to mass produce the gan bulk crystal substrate for use in various optical semiconductor devices including a laser diode and a light emitting diode fig1 shows an example of the seed crystal 701 provided at the tip end of the rod 702 referring to fig1 the seed crystal 702 is formed to have a slab shape with a width w and a thickness d corresponding to the width and thickness of the gan substrate to be formed thus by pulling up the rod 702 straight in the upward direction a slab shaped gan bulk crystal is grown continuously thus by merely polishing the surface of the gan bulk crystal slab followed by a cleaving process it is possible to mass produce the gan bulk crystal substrate having a quality suitable for use in various optical semiconductor devices including a laser diode and a light emitting diode as other features of the present embodiment is more or less the same as those of the previous embodiments further description thereof will be omitted fig1 shows the construction of a growing apparatus 800 according to an eighth embodiment of the present invention wherein those parts.