A liquid crystal compound selected from a group of compounds represented by formula (a): ##STR00001## wherein Ra and Rb are each independently hydrogen, alkyl having 1 to 10 carbons or alkenyl having 2 to 10 carbons, provided that in the alkyl, --CH.sub.2-- may be replaced by --O--, but plural --O-- are not adjacent to each other, and hydrogen may be replaced by fluorine; ring A.sup.1 and ring A.sup.2 are each independently trans-1,4-cyclohexylene or 1,4-phenylene, provided that one or two hydrogens of the 1,4-phenylene may be replaced by halogen, and in a 6-membered ring of these groups, one --CH.sub.2-- or two --CH.sub.2-- that are not adjacent to each other may be replaced by --O--, and one or two --CH.dbd. may be replaced by --N.dbd.; Z.sup.1 and Z.sup.2 are each independently a single bond, --(CH.sub.2).sub.2--, --(CH.sub.2).sub.4--, --CH.dbd.CH--, --C.ident.--, --CH.sub.2O--, --OCH.sub.2--, --COO--, --OCO-- or --OCF.sub.2--; l and m are each independently 0, 1 or 2, provided that l+m is 0, 1, 2 or 3; and n is an integer of from 0 to 6, provided that in --(CH.sub.2).sub.n--, --CH.sub.2-- may be replaced by --O--, but plural --O-- are not adjacent to each other, and hydrogen may be replaced by fluorine.