At CRYPTO 2016, Kim and Barbulescu proposed an efficient number field sieve (NFS) algorithm for the discrete logarithm problem (DLP) in a finite field. The security of pairing-based cryptography (PBC) is based on the difficulty in solving the DLP. Hence, it has become necessary to revise the bitlength that the DLP is computationally infeasible against the efficient NFS algorithms. The timing of the main operations of PBC (i.e. pairing, scalar multiplication on the elliptic curves, and exponentiation on the finite field) generally becomes slower as the bitlength becomes longer, so it has become increasingly important to compute the main operations of PBC more efficiently. To choose a suitable pairing-friendly curve from among various pairing-friendly curves is one of the factors that affect the efficiency of computing the main operations of PBC. We should implement the main operations of PBC and compare the timing among some pairing-friendly curves in order to choose the suitable pairing-friendly curve precisely. In this paper, we focus on the five candidate pairing-friendly curves from the Barreto- Lynn-Scott (BLS) and Kachisa-Schaefer-Scott (KSS) families as the 256- bit secure pairing-friendly curves and show the following two results; (1) the revised bitlength that the DLP is computationally infeasible against the efficient NFS algorithms for each candidate pairing-friendly curve, (2) the suitable pairing-friendly curve by comparing the timing of the main operations of PBC among the candidate pairing-friendly curves using the revised bitlength.