Performance measurement of the particle-in-cell (PIC) method for collisionless plasma is made on the strong scaling of the thread-level parallelism with OpenMP. The conventional program structure of the PIC method, in which a single loop statement involves an iteration through the list of particles, is compared with the new program structure, in which outer multiple loop statements involve iterations through spatial grid cells and the most inner single loop statement involves an iteration through the list of particles. The present strong scaling measurement shows that the new program structure improves both performance and scalability of the PIC code from the conventional program structure. The new code runs about three times faster than the conventional code without sorting of the list of particles.