An organoaluminum compound represented by the formula (i) R1.sbsp.m Al(OR2)n.Hp Xq wherein each of R1 and R2 is a hydrocarbon group having usually 1-15 carbon atoms, preferably 1-4 carbon atoms, and they may be the same or different from each other; X is halogen; m, n, p and q are numbers satisfying the conditions of 0???m???3, 0???n???3, 0???p???3, 0???q???3, and m+n+p+q=3.