  We extend the concept of the negativity, a good measure of entanglement for
bipartite pure states, to mixed states by means of the convex-roof extension.
We show that the measure does not increase under local quantum operations and
classical communication, and derive explicit formulae for the entanglement
measure of isotropic states and Werner states, applying the formalism presented
by Vollbrecht and Werner [Phys. Rev. A {\bf 64}, 062307 (2001)].
