The tunneling-rotation transitions of malonaldehyde, 43 a-type Q-branch lines, were observed in the submillimeter-wave region of 643-651 GHz. The proton tunneling splitting in the ground state, Delta E-0 = 647046.208 +/- 0.019 MHz, and the interaction constant, F = 45.8965 +/- 0.0082 MHz, were determined, as well as the molecular constants for each tunneling sublevel.