We have succeeded in the first measurement of the sound velocity of lysozyme single crystals using an ultrasonic pulse-echo method. The sound velocity along the [110] crystallographic direction in the tetragonal crystals is obtained to be 1817 m/s in the solution with pH 4.3 at 20 degreesC. The corresponding elastic constant C-11 + C-66 is evaluated to be approximately 3.99 GPa. Assuming the isotropic body, the dynamic Young modulus is estimated to be 2.70 GPa. The temperature and pH dependences of the sound velocity are also observed. Their mechanisms are discussed in relation to the inter-molecular contacts in the crystal.