This invention relates to an algicide of excellent water solubility, low cost and excellent handling that is characterized in containing N-chloro-2-pyrrolidinone, as well as to an algicidal/microbicidal agent characterized in containing one or more compounds selected from cyclic N-chloro compounds and N,N-dichloro compounds represented by formula (1). (1) (In the formula, X represents a methylene group, secondary amino group, or N-chloro group, and R represents a C2-5 alkylene group, provided that when X is an N-chloro group, R is a C2-3 alkylene group.)本發明之課題在於提供一種在水中之溶解性優異、低成本且操作性優異之殺藻劑及殺藻殺菌劑。本發明係關於一種殺藻劑及殺藻殺菌劑,該殺藻劑之特徵在於:其含有N-氯-2-吡咯烷酮,該殺藻殺菌劑之特徵在於:其含有選自下述式(I)所示之環狀N-氯化合物或N,N-二氯化合物中之一種以上。
