Hydraulic pressure control unit, brake system, and diagnosis method

A diagnosis method of a hydraulic pressure control unit (5) that is used in a brake system (10) of a straddle-type vehicle (100),

• the hydraulic pressure control unit (5) including:
• a hydraulic pressure control mechanism (51) including an inlet valve (31) as an electromagnetic valve that is electrically connected to a power supply (7) through a power supply line (8), that is provided to a flow path (25) that communicates with a master cylinder (21) and a wheel cylinder (24), and that has an open state in no-energization and has a closed state in energization;
• and
• a control device (52) that controls an operation of the hydraulic pressure control mechanism (51),
• and further including a resistance (35) that is electrically connected to the power supply (7) through the power supply line (8),
• the diagnosis method comprising performing, by a diagnosis unit (52c) of the control device (52), a power supply line diagnosis in which an abnormality of the power supply line (8) is diagnosed, in a state where no current is applied to the inlet valve (31) but a current is applied to the resistance (35), based on a voltage change in the power supply line (8) when the current to be applied to the resistance (35) is changed.