PURPOSE: To prevent the occurrence of such a serious fault where the lock state or the data of a subsystem is transformed even with the insufficient margin or an intermittent fault of a component device.

CONSTITUTION: When an error is detected, it is decided whether the error area is included in a storage director (110) or a disk adaptor (114). Then the value of the corresponding counter is increased by 1 (112, 115, 118) and the error occurring frequency is counted for each component device of a subsystem. These operations are carried out every time an error is detected. When the value of the counter exceeds a certain threshold level, a suppression control indicator is set (113, 117, 120). A microprogram refers to the control indicator after reception of an input/output instruction. Then the microprogram becomes idle without giving any answer to the input/output instruction as long as the control indicator is set.