A fuel injection pump for high pressure injecting gasoline, fuel whose main component is gasoline or light oil having a sulfur content of 0.05% by weight or less by a reciprocation of a piston sliding relative to a cylinder, wherein at least a sliding surface of the piston to slide on the cylinder is made of ceramic and the surface roughness thereof, expressed in ten-point mean surface roughness Rz, is 0.05-0.4 &mgr;m in a direction perpendicular to the sliding direction and 0.2-0.6 &mgr;m in a direction parallel to the sliding direction while the surface roughness expressed in the ten-point mean surface roughness Rz of a cylinder sliding surface corresponding thereto is 0.2-0.8 &mgr;m in a direction parallel to the sliding direction. By using the above combination of piston and cylinder, the wear resistance of the piston and cylinder is improved, the durability thereof is improved and the production cost can be largely reduced.