Temperature dependence of the sound velocity of a single crystalline La_Sr_CuO_4 has been measured in high magnetic fields. An increase of the sound velocity due to the flux line lattice (FLL) pinning is observed at a temperature below the superconducting transition temperature T_C. Anisotropic activation energies to depin flux lines are evaluated separately from measurements under various settings of the directions of wave vector k, polarization vector u and magnetic fields H using the analysis based oh the thermally assisted flux flow model. Besides the FLL elasticity, anomalous temperature dependence of elastic constants of c_ and (c_-c_)/2 (softening at low temperatures and hardening at lower temperatures below 10K) is observed in high magnetic fields