1. A management computer comprising a memory storing a program and a CPU (central processing unit) refering to the program, wherein, by the program, the CPU executes to: store a first rule including first conditions to be satisfied for indicating a first cause; receive event messages regarding events related to a plurality of monitored nodes, wherein the events include a first event and a second event; assign a first valid duration to a first one of the first conditions, which corresponds to the first event; assign a second valid duration to a second one of the first conditions, which corresponds to the second event; and calculate a first matching ratio indicating an accuracy of a conclusion acquired by the first rule, based on the first valid duration and the second valid duration.