Pressure-reducing valve, valve unit, valve device, and leaf spring

A pressure-reducing valve comprising:

• a casing in which a valve passage is formed;
• a valve body that is movably housed in the casing, receives a primary pressure in an opening direction in which the valve passage opens, and changes a position thereof according to a secondary pressure to adjust an opening degree of the valve passage;
• and
• a biasing member that receives the secondary pressure, moves the valve body to a position corresponding to the secondary pressure received, and biases the valve body against the secondary pressure in the opening direction, wherein:
• the biasing member is a spring in the shape of a circular disc and extends laterally from the valve body.