A power source system includes: a rotary electrical machine; a power source; a switch that is provided between the power source and the rotary electrical machine; and a power source control device that controls opening/closing of the switch in response to a request for power feeding to the rotary electrical machine. The power source control device is configured to, in response to the power source system stopping operation, bring the switch into a closed state in a predetermined period of time. A rotary electrical machine control device controlling the rotary electrical machine includes: a determination unit that, in response to the power source system stopping operation, determines that the switch is in the closed state; and an abnormality diagnosis unit that, in response to the determination unit determining that the switch is in the closed state, performs an abnormality diagnosis of the rotary electrical machine.