Provided is anin vivoimaging agent including trans-1-amino-[18F]fluorocyclobutanecarboxylic acid or a pharmaceutically acceptable salt thereof as an active ingredient, which is used for imaging a metastatic brain tumorin vivo.本發明係為一種用來對轉移性腦腫瘤進行生體內造影,且以反式-1-氨基-[18F]氟環丁烷羧酸或藥學上可接受之鹽為有效成分之生體內成像劑。