A charged particle beam writing apparatus includes a first limiting aperture member, in which a first opening is formed, to block a charged particle beam having been blanking-controlled to be beam “off”, and to let a part of the charged particle beam having been blanking-controlled to be beam “on” pass through the first opening, a first detector to detect a first electron amount irradiating the first limiting aperture member, in a state where beam “on” and beam “off” are repeated, a first integration processing unit to generate a first integrated signal by integrating components in a band sufficiently lower than a band of a repetition cycle of beam “on” and beam “off”, in a first detected signal detected for obtaining the first electron amount, and a first irregularity detection unit to detect irregularity in a dose amount of the charged particle beam by using the first integrated signal.