  We study how entanglement among the register qubits affects the gate fidelity
in the one-way quantum computation if a measurement is inaccurate. We derive an
inequality which shows that the mean gate fidelity is upper bounded by a
decreasing function of the magnitude of the error of the measurement and the
amount of the entanglement between the measured qubit and other register
qubits. The consequence of this inequality is that, for a given amount of
entanglement, which is theoretically calculated once the algorithm is fixed, we
can estimate from this inequality how small the magnitude of the error should
be in order not to make the gate fidelity below a threshold, which is specified
by a technical requirement in a particular experimental setup or by the
threshold theorem of the fault-tolerant quantum computation.
