The present invention provides a composition for suppressing the expression of a KRAS gene, comprising a lipid particle containing, as a drug, a double-stranded nucleic acid having an antisense strand having a sequence of bases complementary to the sequence of at least 19 continuous bases of any one KRAS gene's mRNA of sequence Nos. 1 to 3； and a cationic lipid represented by the following formula (I): wherein R 1 and R 2 , which are the same or different, are each linear or branched alkyl, alkenyl or alkynyl having a carbon number of from 12 to 24； L 1 and L 2 , which are the same or different, are each -CO-O- or -O-CO-； a and b, which are the same or different, are each 1 to 3； and R 3 is a hydrogen atom, alkyl having a carbon number of from 1 to 6, or alkenyl having a carbon number of from 3 to 6, and the like.
