The objective of this paper is to reveal the attenuation characteristics of a shock wave after optical breakdown in water, with laser pulses of 12-ns duration. A high time-resolved shadowgraph method is applied to capture the temporal evolutions of the cavitation bubble wall and shock wave. The experiments are carried out on a single bubble generated far away from the free surface and the rigid walls with laser pulse energies of 22 mJ, 45 mJ, and 60 mJ. The results show that a high, time-resolved, wave front velocity of the shock wave is identified, and the maximum velocity can reach up to around 4000 m/s. An asymmetric shock wave is observed at the very start of the bubble expansion stage, and the process of the sharp attenuation of wave front velocity down to sound velocity is accomplished within 310 ns. The possible relationship of the cavitation bubble and the shock wave is discussed and a prediction model, using the maximum bubble radius and the corresponding time calculated by the Gilmore model, is proposed to calculate the location of the wave front.