There is disclosed a photocleavable sense-antisense nucleobase polymer complex capable of modulating gene expression comprising an unnatural antisense nucleobase polymer that targets an mRNA, and a photocleavable sense nucleobase polymer noncovalently bound to the antisense nucleobase polymer, wherein the photocleavable sense nucleobase polymer comprises a plurality of nucleobase polymers connected by a photocleavable linkage. There is also disclosed a method for controlling the time and spatial position of gene expression comprising selecting a target mRNA, introducing the photocleavable sense-antisense nucleobase polymer complex into a cell, and selectively irradiating the cell with light.