Method for molding fiber-reinforced plastic, and molding device for same

A method for molding fiber-reinforced plastic. A core is formed in a desired shape by accommodating, in a flexible bag, a grain group containing plurality of grains. The core is placed inside a prepreg containing resin and fibers, and the prepreg, in which the core is housed is placed in a molding die and compression molded. When doing so, the grain group contains first and second grains (a,b) that satisfy the equation (1). (1) 1.1□(Da/Db)□2.0 In the equation Da is the grain diameter of the grains (a), and Db is the grain diameter of the grain (b). When using a molding die to mold a molded article having a cavity, the above mentioned molding method enables an increase in the internal pressure of the core in order to change the peripheral surface area of the core, without using a pressurized gas and/or pressurized liquid.