FIGS. 1 to 5 are longitudinal sectional views showing a first embodiment of the lancet instrument according to the present invention, FIGS. 6 and 7 are views (longitudinal views) for illustrating the method of using the lancet instrument (first embodiment) according to the invention by fitting it to a lancing apparatus, and FIGS. 8A and 8C are end views and FIG. 8B is a side view showing the confi