In this study, we developed elastic-constant measurement in thin films at low temperatures using picosecond laser ultrasound spectroscopy. Specimens are cooled with liquid He through a heat exchanger in a cryostat, and an ultrahigh-frequency acoustic pulse is generated using a femtosecond light pulse, which propagates in the film-thickness directioa Acoustic-phonon resonances are observed by the changes in reflectivity of the time-delayed probe light and give the longitudinal-wave out-of-plane elastic constant We observed the temperature dependence of elastic constant C^<111> of a Pd thin film. This method is useful for studying the elastic properties of metallic thin films at cryogenic temperatures.