The present invention provides a composition containing a single-stranded nucleic acid molecule consisting of a nucleotide sequence shown by 5′-AGCAGAGUACACACAGCAUAUACC-P-GGUAUAUGCUGUGUGUACUCUGCUUC-P-G-3′ (SEQ ID NO: 1)(in the sequence, P is a proline derivative linker represented by (I) in the DESCRIPTION) and a buffer, and having the following features: (a) being in the form of a solution at ambient temperature; and (b) a content of the nucleic acid molecule after storage at 25° C., relative humidity 60% for 4 weeks, of not less than 80% relative to the content at the time of start of the storage.