The present invention provides a composition for suppressing the expression of the CKAP5 gene, the composition comprising a lipid particle containing a double-stranded nucleic acid as a drug and a cationic lipid, the double-stranded nucleic acid having an antisense strand having a base sequence complementary to the sequence of at least 19 continuous bases in CKAP5 gene mRNA of any one of SEQ ID NOs: 1 to 6, the cationic lipid being represented by formula (I): wherein R 1 and R 2 are the same or different and are each linear or branched alkyl, alkenyl or alkynyl of 12 to 24 carbon atoms L 1 and L 2 are the same or different and are each -CO-O- or -O-CO- a and b are the same or different and are each 1 to 3 and R 3 is a hydrogen atom, alkyl of 1 to 6 carbon atoms, or alkenyl of 3 to 6 carbon atoms a medicament comprising the composition, and the like.
