A lancing device (A) includes an auxiliary member (8) located on a side of the tip end portion of a lancet (L). By moving the auxiliary member (8), the dimension (s) of the tip end of the lancet (L) projecting from the tip end portion (8a) of the auxiliary member (8) can be varied. The variation of the dimension (S) makes it possible to stick the lancet (L) into the skin (99) of a user precisely by an intended amount.