Abstract: A rubber syringe piston in a generally cylindrical shape slidably mounted in a syringe barrel. The piston has a circular depression at an intermediate part thereof. R1 is smaller than R2 where sliding surfaces are disposed on the both sides of the circular depression and R1 is a diameter of the sliding surface on the leading end side of the piston and R2 is a diameter of the sliding surface on the rear end side of the piston, L1 is equivalent to or larger than L2 in the case in which L1 is a length of the sliding surface on the leading end side of the piston and L2 is a length of the sliding surface on the rear end side of the piston, and r is equivalent to or larger than 0.7 mm where r is a radius of curvature of a corner portion on the leading end side of the piston.