A highly efficient joining method using ultrasonic welding is necessary for manufacturing and processing thermoplastic carbon fibre-reinforced plastic (CFRP) structures. The application of resin mesh as a sheet-like energy director is proposed, and its feasibility is assessed. The relationship between the welding conditions and the single-lap shear strength was investigated using specimens prepared by ultrasonic welding. The fracture surface observations indicated that resin mesh melted from the edges of the specimen, whereas the unmelted regions remained at the centre of the welding surface. It was necessary to supply a sufficient amount of resin to the welding surface and select a resin mesh suitable for the welding conditions to minimize variations in the bonding strength. Under the welding conditions used herein, the specimens with the coarsest resin mesh showed an average single-lap shear strength of 34 MPa and a coefficient of variation of 0.1. The study proves that the application of a sheet-like energy director for the ultrasonic welding of thermoplastic CFRP is a promising method to achieve practical bonding strength.