To understand the physics of double-pulse femtosecond laser ablation of metals, we have proposed and implemented a new ablation method for measuring the temporal change in the laser penetration length (LPL). The measurements were performed for titanium and platinum with femtosecond laser pulses with a central wavelength of 810 nm, a repetition rate of 10 Hz, and a pulse duration of 45 fs. The delay between the seed pulse and the ablation pulse was between 0.3 ps and 1 ns for titanium and between 0.3 ps and 30 ns for platinum. A low-fluence (below the ablation threshold) seed pulse and a high-fluence (above the ablation threshold) ablation pulse were used for the measurement. The seed pulse modifies the optical properties of the target surface, and the ablation pulse creates a crater on the modified surface. The LPL after seed-pulse irradiation was estimated by analyzing how the ablation rate depends on the laser fluences. With a delay of 300 ps, at which the ablation rate is the lowest for both titanium and platinum, the LPL for a titanium target was 40% of that for the target without seed-pulse irradiation, while the LPL for a platinum target was almost the same as that for the target without seed-pulse irradiation. The reduced LPL corresponded to suppressed ablation for titanium and platinum with a delay of 300 ps. The suppression ratio 𝜂sup (ablation rate with seed pulse divided by ablation rate without seed pulse) for titanium (𝜂sup= 0.5) was less than that for platinum (𝜂sup= 0.8). The LPL measured using our proposed ablation method is a key parameter changed by seed-pulse irradiation. For platinum with a delay exceeding 1 ns, a characteristic increase of LPL was also measured