Rare earth tetraborides have a tetragonal crystalline structure with the space group P4/mbm. The rare earth ions form the Shastry-Sutherland lattice in the c plane in these compounds, which evokes us an interesting electronic property due to the geometric frustration of spins and orbitals. TbB4 undergoes two phase transitions at TN1 = 42.1 K and TN2 = 21.7 K, respectively. Because of the clear cusp-type anomaly in the temperature dependence of magnetic susceptibility, the transition at TN1 should be an antiferromagnetic one. The origin of the transition at TN2 is unclear at present. To investigate the possibility of multipolar ordering, we measured temperature dependencies of elastic moduli using the ultrasonic phase comparison method. The longitudinal elastic modulus C11 shows large softening with more than 17% reduction of the stiffness at TN2 in contrast to no anomaly at TN1. This result suggests ferro-type ordering of the quadrupole OΓ3 at TN2. The macroscopic spontaneous strain in the c plane emerging due to the ferroquadrupolar ordering may lift the geometric frustration.