An impulse current response to a short laser pulse was measured using a current-voltage converter with a zero input resistance, which enabled us to provide any band bending of a semiconductor electrode under a potentiostatic condition. The current transient can monitor a net charge-transfer process across a semiconductor-electrolyte interface because the external resistance is almost zero. The current transient was found to be characterized by a relaxation time tau = (R(dl) + R(soi))C-sc where R(dl), R(sol), and C-sc are the diffuse double-layer resistance, the solution resistance, and the depletion layer capacitance of the semiconductor. C-sc obtained from the analysis of the current transient gave an ideal Mott-Schottky plot. By intentionally increasing the diffuse layer thickness with which R(dl) and then tau increase, the dynamics of protons generated as an oxidation product of water was confirmed to be monitored from the current transient.