This free-cutting copper alloy contains 75.0%-78.5% Cu, 2.95%-3.55% Si, 0.07%-0.28% Sn, 0.06%-0.14% P, and 0.022%-0.25% Pb, with the remainder being made up of Zn and inevitable impurities. The composition satisfies the following relations: 76.2≤f1=Cu+0.8×Si−8.5×Sn+P+0.5×Pb≤80.3, 61.5≤f2=Cu−4.3×Si−0.7×Sn−P+0.5×Pb≤63.3. The area ratios (%) of the constituent phases satisfy the following relations: 25≤κ≤65, 0≤γ≤1.5, 0≤β≤0.2, 0≤μ≤2.0, 97.0≤f3=α+κ, 99.4≤f4=α+κ+γ+μ, 0≤f5=γ+μ≤2.5, 27≤f6=κ+6×γ1/2+0.5×μ≤70. The long side of the γ phase does not exceed 40 μm, the long side of the μ phase does not exceed 25 μm, and the κ phase is present within the α phase.