PROBLEM TO BE SOLVED: To provide a compound with a high tumor accumulation rate or a pharmaceutically acceptable salt thereof, and a contrast medium for optical imaging.SOLUTION: The present invention provides a compound having polyethylene glycol covalently linked with at least one specific organic dye, or a pharmaceutically acceptable salt thereof.SELECTED DRAWING: None【課題】腫瘍集積率が高い化合物又はその医薬上許容しうる塩、及び光学イメージング用造影剤を提供する。【解決手段】ポリエチレングリコールに少なくとも1つの特定の有機色素が共有結合した化合物又はその医薬上許容しうる塩。【選択図】なし