PURPOSE: To obtain a new octadecapeptide having both of action of promoting release of insulin and action of suppressing release of glucagon and useful as an agent, etc., for treating diabetes mellitus.

CONSTITUTION: The octadecapeptide expressed by formula I or its salt. Furthermore, this octadecapeptide of formula I is obtained by condensing an undecapeptide fragment of formula II with a heptapeptide fragment of formula III in an inert solvent such as THF, in the presence of an organic base such as triethylamine using a condensing agent such as 1-ethyl-3-(3- dimethylaminopropyl)carbodiimide. A daily dose of the octadecapeptide (salt) is preferably 0.001-5mg/kg weight.