PURPOSE: To obtain an improver which, when added in a small amount, is more effective in heightening cetane value.

CONSTITUTION: The improver comprises as an active ingredient at least one azoalkane derivative represented by formula 1 [wherein R1 and R2 may be the same or different and each is an aryl, phenoxy, a lower alkyl having at least one substituent selected among lower alkoxys and halogens, or phenyl which may have at least one substituent selected among lower alkyls and halogens, provided that R1 and R2 may form a cycloalkyl group in cooperation with the carbon atom bonded thereto; and R3 is a lower alkyl, an aryloxy,- OCOR4 (where R4 is a lower alkyl), a halogen atom, isocyanato, phenoxy, benzyloxy, aminocarbonylamino, or a lower alkoxycarbonylamino].