Abstract: The present invention provides a compound or a pharmaceutically acceptable salt thereof containing a modified oligonucleotide with a length of 8 to 80 consecutive nucleosides, in which the modified oligonucleotide has a nucleobase sequence containing at least 8 consecutive nucleobases contained in a nucleobase sequence of any one of SEQ ID NOs: 3 to 73. With the compound or a pharmaceutically acceptable salt thereof, it is possible to treat a disease or a condition against which inhibition of CALM2 gene expression by controlling of the CALM2 gene expression is effective (particularly, congenital long QT syndrome).