1. A compound of the following formula (I): ##STR6## wherein: m is an integer of 1 to 3; R.sub.1 and R.sub.2 are the same or different and each is hydrogen atom or lower alkyl; and R.sub.3, R.sub.4, R.sub.5 and R.sub.6 are the same or different and each is hydroxy or a group of the formula ##STR7## wherein n is 0 or 1; X is --NH-- or --O--; Y is alkylene; A is hydrogen atom, lower alkyl, lower alkoxy, halogen atom or trifluoromethyl; and B is alkyl or alkenyl, the total carbon number of Y and B is 8-12, with the proviso that two or three of R.sub.3, R.sub.4, R.sub.5 and R.sub.6 are hydroxyl groups and that when two of them are hydroxyl groups, the cases where R.sub.3 and R.sub.5 are hydroxy, and R.sub.4 and R.sub.6 are hydroxy are excluded; or a salt thereof.