The present invention establishes a molecular therapy for glycogen storage disease type Ia. The present invention provides an oligonucleotide of 15-30 bases comprising a nucleotide sequence complementary to die cDNA of G6PC gene with c.648G>T mutation, wherein the oligonucleotide comprises a sequence complementary to a region comprising any site between the 82nd to the 92nd nucleotide from the 5′ end of exon 5 of the G6PC gene with c.648C>T mutation, a pharmacologically acceptable salt or solvate thereof. Also provided is a pharmaceutical drug comprising the oligonucleotide, a pharmacologically acceptable salt or solvate thereof (e.g., therapeutic drug for glycogen storage disease type Ia).