A substantially solvent-free and stable crystal of the compound of the formula: ##STR1## wherein the ring A may optionally be substituted, R.sup.1 represents hydrogen or an N-protecting group, each of R.sup.2, R.sup.3 and R.sup.4 (1) a hydrogen atom, (2) an alkyl group which may optionally be substituted with halogen atom(s) or (3) an alkoxy group which may optionally be substituted with halogen atom(s) or alkoxy; or its salt, is produced by subjecting a solvate of the compound (I) or its salt to de-solvent treatment, in an industrially advantageous method.