We have performed a pump and probe experiment for real-time reflectivity measurements as a function of superimposed laser shots at the fluence below the ablation threshold to induce nanostructure formation on thin films of diamond-like carbon (DLC). The evolution of reflectivity resolved the bonding structure change and the nanostructure formation through coherent interaction between the probe pulse and the film surface excited with the pump pulse. The characteristic reflectivity changes observed show that the nanostructure formation is preceded by the bonding structure change to swell the film surface. Based on the experimental results, we discuss the ultrafast dynamics of the nanostructure formation.