  We propose the bosonic part of an action that defines M-theory. It possesses
manifest SO(1, 10) symmetry and constructed based on the Lorentzian 3-algebra
associated with U(N) Lie algebra. From our action, we derive the bosonic sector
of BFSS matrix theory and IIB matrix model in the naive large N limit by taking
appropriate vacua. We also discuss an interaction with fermions.
