  To implement fault-tolerant quantum computation with continuous variables,
the Gottesman--Kitaev--Preskill (GKP) qubit has been recognized as an important
technological element. We have proposed a method to reduce the required
squeezing level to realize large scale quantum computation with the GKP qubit
[Phys. Rev. X. {\bf 8}, 021054 (2018)], harnessing the virtue of analog
information in the GKP qubits. In the present work, to reduce the number of
qubits required for large scale quantum computation, we propose the tracking
quantum error correction, where the logical-qubit level quantum error
correction is partially substituted by the single-qubit level quantum error
correction. In the proposed method, the analog quantum error correction is
utilized to make the performances of the single-qubit level quantum error
correction almost identical to those of the logical-qubit level quantum error
correction in a practical noise level. The numerical results show that the
proposed tracking quantum error correction reduces the number of qubits during
a quantum error correction process by the reduction rate
$\left\{{2(n-1)\times4^{l-1}-n+1}\right\}/({2n \times 4^{l-1}})$ for $n$-cycles
of the quantum error correction process using the Knill's $C_{4}/C_{6}$ code
with the concatenation level $l$. Hence, the proposed tracking quantum error
correction has great advantage in reducing the required number of physical
qubits, and will open a new way to bring up advantage of the GKP qubits in
practical quantum computation.
