In order to make the information systems secure, a ZT (Zero Trust) model is proposed and discussed. Since it is critical to make the shift from the present system model to the ZT model smooth, a TZ (Trust Zone) model is proposed. Here, every object is in a trust zone and an authorization decision is made for each trust zone. However, prevention of the illegal information flow is not discussed. Hence, a TZMAC (TZ with Mandatory Access Control) model is proposed to prevent illegal information flow in our previous studies. Here, subjects also belong to trust zones. It is decided whether or not accesses by subjects are allowed in the criteria based trust algorithm. However, it is difficult to realize the dynamic access control in the criteria based trust algorithm. In this paper, we newly propose the score based trust algorithm. Here, the score means how much the behavior of a subject is trustworthy. The current behavior trust is decided based on not only the behavior trust on the access but also the previous behavior trust of the subject. If a subject issued operations implying illegal information flow previously, the current behavior trust of the subject is decided to be smaller. If the current behavior trust is larger than the trust threshold configured for an object, the access is allowed.