A drilling device for a down-the-hole drilling apparatus. The drilling device includes in a guide, a drilling member and a flushing mechanism configured to flush drilling waste. The drilling mechanism includes first and second drilling bit arrangements that are movable with respect to the drilling device and that are arranged to be expanded to drill a hole having a greater cross-sectional surface area than an end of the casing and arranged to be reduced to a size smaller than the end of the casing. Bit pieces of the second drill bit arrangement expand and reduce the second drill bit arrangement. The bit pieces expand the second drill bit arrangement by moving in a radial direction outwards in slide grooves, by rotating the guide in a drilling direction, and move in the radial direction inwards in the slide grooves, by rotating the guide in an opposite direction.