  The operational dc-SQUID based on intrinsic Josephson junctions in
Bi$_2$Sr$_2$CaCu2O$_{8+\delta}$ high-$T_c$ superconductor is fabricated and
studied. The novel in-plane loop layout and the developed in-situ endpoint
detection method allowed an accurate control of the number of junctions in the
SQUID. A clear periodic modulation of the superconducting current as a function
of magnetic flux through the SQUID loop is observed. This is an unambiguous
evidence for the quantum interference phenomenon in
Bi$_2$Sr$_2$CaCu2O$_{8+\delta}$ single crystals.
