  An optimal effective current operator for flux qubit has been investigated by
taking account of the inductive effects of the circuit loop. The whole system
is treated as two interacting subsystems: one is the inductance-free flux qubit
consisting of three Josephson junctions and the other a high frequency
LC-oscillator. As the composite system hardly affords one excessively high
energy LC photon, an effective theory for the inductive flux qubit providing
its physical variable operators has been achieved, which can take account of
the inductive effects but does not include the additional degree of freedom for
the LC-oscillator. Considering the trade-off between simplicity and accuracy,
it has been revealed that the optimal effective current operator resulting in
an error only on the order of $L^{3/2}$ provides an approximation of high
accuracy, which is also verified numerically.
