To provide a fault detection device for an injector capable of detecting a fault such as close adhesion in the injector.

This fault detection device for the injector for detecting a fault caused in the injector 20 used in an internal combustion engine for gas fuel detects that the fault occurs in the injector based on a characteristic value related to opening and closing operation of the injector in a first condition in which the injector is driven by predetermined driving force and a characteristic value related to opening and closing operation of the injector in a second condition in which driving force of the injector is increased more than in the first condition. When the characteristic value related to the opening and closing operation of the injector is greatly changed (improved) due to increase in driving force of the injector when compared with a condition before driving force of the injector is increased, occurrence of the fault in the injector is detected.

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