A rock drilling device intended to be used for mounting a drill bit on a drill rod comprising: - a first arm freely movably attached to a base unit, - a first drill unit freely movably attached to said first arm, wherein the first drill unit comprises said drill rod, a frame and a feeding device for axial displacement of said drill rod along the axis of rotation of the drill rod (Ab) relative to said frame, - a holder fixedly mounted to a support and configured to hold at least two drill bits, wherein a free movement of said first drilling unit in relation to said holder is permitted, and wherein the holder comprises a housing and an inner part adapted to receive at least two drill bits, wherein said inner part is adapted to hold said at least two drill bits at transport and to permit mounting of each drill bit on a drill rod, wherein the inner part is arranged to counteract a rotational movement and to permit an axial displacement of the drill bits in relation to said inner part upon mounting of these on said drill rod, and - a control unit arranged to: align the first drill unit, by said first arm against said drill bit located in the holder so that the axis of rotation of the drill rod (Ab) substantially coincides with a rotation axis (Ak) of the drill bit, displace said drill rod axially forward towards the drill bit to a first position of said drill rod permitting mounting of the drill bit, attach the drill bit to the drill rod and displace said drill rod axially backward to a second position in which said drill bit has been removed from said holder. - a first arm freely movably attached to a base unit, - a first drill unit freely movably attached to said first arm, wherein the first drill unit comprises said drill rod, a frame and a feeding device for axial displacement of said drill rod along the axis of rotation of the drill rod (Ab) relative to said frame, - a holder fixedly mounted to a support and configured to hold at least two drill bits, wherein a free movement of said first drilling unit in relation to said holder is permitted, and wherein the holder comprises a housing and an inner part adapted to receive at least two drill bits, wherein said inner part is adapted to hold said at least two drill bits at transport and to permit mounting of each drill bit on a drill rod, wherein the inner part is arranged to counteract a rotational movement and to permit an axial displacement of the drill bits in relation to said inner part upon mounting of these on said drill rod, and - a control unit arranged to: align the first drill unit, by said first arm against said drill bit located in the holder so that the axis of rotation of the drill rod (Ab) substantially coincides with a rotation axis (Ak) of the drill bit, displace said drill rod axially forward towards the drill bit to a first position of said drill rod permitting mounting of the drill bit, attach the drill bit to the drill rod and displace said drill rod axially backward to a second position in which said drill bit has been removed from said holder. align the first drill unit, by said first arm against said drill bit located in the holder so that the axis of rotation of the drill rod (Ab) substantially coincides with a rotation axis (Ak) of the drill bit, displace said drill rod axially forward towards the drill bit to a first position of said drill rod permitting mounting of the drill bit, attach the drill bit to the drill rod and displace said drill rod axially backward to a second position in which said drill bit has been removed from said holder.