PURPOSE: To enable prediction in a short time by predicting performance at the time of a job quantity fluctuation and of hardware configuration change based on calculated average response and processing ability information in unit time by means of hardware configuration.

CONSTITUTION: A performance predicting device is constituted of a first processing means 14 which collects working information 11 from a computer system so as to obtain average response at the time of the job quantity fluctuation based on the working information 11 and a second processing means 12 which predicts performance at the time of the job quantity fluctuation and of hardware configuration change based on calculated average response and processing ability information in unit time by fardware configuration given for predicting performance. Then, working information 11 of the computer system is collected, for example, at prescribed time, average response at the time of the job quantity fluctuation is obtained based on the working information 11 and performance at the time of the job quantity fluctuation and of hardware configuration change is predicted based on calculated average response and processing ability information in unit time by hardware configuration given for predicting performance.