A novel pyrrolealdehyde derivative represented by the following formula (I): ##STR1## wherein R represents C.sub.10 -C.sub.16 alkyl unsubstituted or substituted by at least one substituent selected from the group consisting of halo, hydroxy, amino, carbamoyl, C.sub.1 -C.sub.5 alkylamino, C.sub.2 -C.sub.6 dialkylamino, C.sub.2 -C.sub.6 acylamino, C.sub.1 -C.sub.5 alkylthio, mercapto, C.sub.2 -C.sub.6 acyloxy, carbamoyloxy, C.sub.6 -C.sub.12 aryl and C.sub.3 -C.sub.7 cycloalkyl; or C.sub.10 -C.sub.16 alkenyl having at least one vinyl, and a pharmaceutically acceptable salt thereof are provided. The compounds are highly effective in reducing the level of triglyceride and cholesterol in serum, and useful as an active ingredient of a pharmaceutical composition for treating hyperlipemia and arteriosclerosis.