Chromone derivative, and aldose reductase inhibitor comprising said compound as active component

A chromone derivative or a pharmaceutically acceptable salt thereof, which is represented by Formula (I'):

• ##STR9## wherein R.sub.1 is selected from the group consisting of ethyl and a branched alkyl group represented by Formula II:
• ##STR10## wherein R.sub.8 and R.sub.9 are each selected from the group consisting of methyl and ethyl;
• R.sub.2 is selected from the group consisting of a hydrogen atom, methyl, ethyl, and a branched alkyl group represented by Formula II;
• R.sub.3 represents a sulfur atom;
• R.sub.4 is selected from the group consisting of a hydrogen atom and a lower alkoxy group;
• and R.sub.7 is selected from the group consisting of phenyl, benzyl, pyridyl, benzimidazolyl, benzthiazolyl, thiazolynyl and thiadiazolyl groups which may be substituted by a hydroxy group, a glycyloxy group, a .beta.-aspartyloxy group or a 4-(4-methylpiperazinomethyl) benzoyloxy group.