A lipid membrane structure encapsulating an siRNA inside thereof and containing a lipid compound of the formula (I) as a lipid component (R 1 and R 2 represent CH 3 -(CH 2 ) n -CH=CH-CH 2 -CH=CH-(CH 2 ) m -, n represents an integer of 3 to 5, m represents an integer of 6 to 10, p represents an integer of 2 to 7, and R 3 and R 4 represent a C 1-4 alkyl group or a C 2-4 alkenyl group.