Based on a waveguide made of a subwavelength metal-block array with open slots, a kind of scanning antenna is proposed in this work. Low-frequency spoof surface plasmon polaritons (spoof SPPs) exist in this waveguide and under certain conditions, a leaky mode can be produced. The leaky mode could scan the radiation pencil-beam by angles of 53° when the input frequency changes. The leaky mode is derived from a negative slope dispersion curve of some new transmission modes. As this dispersion curve intersects with the light line, it becomes the leaky mode in high-frequency region. Numerical results reveal that the modal field of the transmission mode resides inside the interior of a hollow block as compared with that in a solid metal block array. Experimental verification of the leaky mode at microwave frequencies has been performed, and the results are found to be consistent with the theoretical analysis.