A method for producing a fiber-reinforced plastic joined body in which a fiber-reinforced molded article containing carbon fibers and a thermoplastic resin and having a joining surface (x), and a thermoplastic resin member having a joining surface (y) are welded via the joining surface (x) and the joining surface (y) by applying ultrasonic to the fiber-reinforced molded article, wherein: a maximum value of tan δ by viscoelastic measurement of the fiber-reinforced molded article is less than 70% of a maximum value of tan δ by viscoelastic measurement of the thermoplastic resin contained in the fiber-reinforced molded article within a range of a glass transition temperature ± 100°C of the thermoplastic resin contained in the fiber-reinforced molded article, at least one of the joining surface (x) and the joining surface (y) has an energy director, an application surface of the fiber-reinforced molded article to which ultrasonic sound is applied is smooth so that the application surface does not have a fluctuation more than the maximum height of energy directors, and the energy director is a mountain-shaped energy director so that the energy director has a vertical cross-sectional shape of such that the cross section decreases from the base toward the top thereof, wherein the energy director satisfies the following to: the mountain-shaped energy director has a convex part in the area from a base up to a top thereof when a straight line is drawn from the base to the center of the top of the shape, wherein the vertical cross-sectional shape of the mountain-shaped energy director is a semicircle or an arc, the mountain-shaped energy director has a transition interval from the top toward the base thereof where an amount of change in an area of a horizontal section of the energy director in the transition interval is 0.1 mm 2 or more per mm in a vertical direction relative to the joining direction, wherein an average fiber length of the carbon fibers is 1 to 100 mm, and wherein the volume ratio of the carbon fibers (Vf) is 10 to 70% by volume. a maximum value of tan δ by viscoelastic measurement of the fiber-reinforced molded article is less than 70% of a maximum value of tan δ by viscoelastic measurement of the thermoplastic resin contained in the fiber-reinforced molded article within a range of a glass transition temperature ± 100°C of the thermoplastic resin contained in the fiber-reinforced molded article, at least one of the joining surface (x) and the joining surface (y) has an energy director, an application surface of the fiber-reinforced molded article to which ultrasonic sound is applied is smooth so that the application surface does not have a fluctuation more than the maximum height of energy directors, the mountain-shaped energy director has a convex part in the area from a base up to a top thereof when a straight line is drawn from the base to the center of the top of the shape, wherein the vertical cross-sectional shape of the mountain-shaped energy director is a semicircle or an arc, the mountain-shaped energy director has a transition interval from the top toward the base thereof where an amount of change in an area of a horizontal section of the energy director in the transition interval is 0.1 mm 2 or more per mm in a vertical direction relative to the joining direction,