Abstract: 1 The present invention relates to a novel fatty acid derivative of formula (I), wherein R1 is acyl group; R2 is acyl(lower)alkyl; R3 is hydrogen, aryl(lower)alkyl, etc.; R4 is acyl(lower)alkyl; and X is —O—, —NH— or formula (II) [wherein R5 is lower alkyl, etc.]; and a pharmaceutically acceptable salt thereof, which is useful as a medicament; the processes for the preparation of said fatty acid derivative or a salt thereof; a pharmaceutical composition comprising said fatty acid derivative or a pharmaceutically acceptable salt thereof; etc.