Provided is a blood purifier able to accurately observe the blood purification treatment state in a continuous manner even if a treatment criterion is changed during blood purification treatment. A blood purifier, which allows real-time observation of the blood purification treatment state on the basis of changes in concentration of drainage determined by a computation means (12), wherein the blood purifier is provided with: a treatment criterion changing means (13) for setting or changing a treatment criterion as desired and detecting in advance the concentration of drainage determined under each treatment criterion using a drainage concentration sensor (5)； and a storage means (14) for storing as first prescribed values the concentrations of the drainage under the respective treatment criteria detected by the drainage concentration sensor (5) in the treatment criterion changing means (13), the first prescribed values corresponding to the respective treatment criteria. If the treatment criterion is changed during blood purification treatment, the computation means (12) determines the change in concentration of drainage on the basis of the first prescribed value corresponding to the post-change treatment criterion among the first prescribed values stored in the storage means (14).La présente invention concerne un purificateur de sang permettant de précisément observer l'état d'un traitement de purification du sang de manière continue même si un critère de traitement est modifié durant le traitement de purification du sang. Un purificateur de sang, qui permet l'observation en temps réel de l'état d'un traitement de purification du sang sur la base des modifications de la concentration du drainage déterminé par un moyen de calcul (12), le purificateur de sang étant doté : d'un moyen de modification de critère de traitement (13) pour définir ou modifier un critère de traitement au besoin et détecter par avance la concentration de drainage déterminée selon chaque critèr
