A pressure-reducing valve includes a housing, a pressure-reducing piston, a driving piston, and a spring mechanism. The housing is provided with a primary port and a secondary port. The pressure-reducing piston is displaceably retained in an interior of the housing. The pressure-reducing piston is displaced to control an opening degree of the primary port and configured to separate the interior of the housing into a primary-pressure space connected to the primary port and a secondary-pressure space connected to the secondary port. The driving piston is displaceably retained in the interior of the housing. The driving piston is displaced to cause the pressure-reducing piston to be displaced in association therewith and has a secondary-pressure receiving surface that receives a secondary pressure from a fluid in the secondary-pressure space. The spring mechanism is configured to exert a spring force against the secondary pressure to at least one of the pistons.