PURPOSE: To secure sealability by increasing the rigidity of a spacer expander which supports upper and lower side rails upward and downward and also to improve followability to a cylinder by reducing the rigidity that stretches side rails in a radial direction, of the spacer expander, in the case of a three piece oil ring consisting of the combination of upper and lower side rails and the segmental annular body space expander formed out of an original plate of a shape steel material.

CONSTITUTION: A spacer expander 10 is formed out of an original plate whose outer periphery side cross section is thick and inner periphery side cross section is thin, and outer periphery side and inner periphery side slits 11, 12 that cut into the spacer expander alternately from the outer periphery side and the inner periphery side, are provided, and the inner periphery side end potions of upper and lower side rails 20, 21 are supported radially by bending alternately upward and downwards spacer expander inner periphery side portions 16a, 16b pinched by the adjoining inner periphery side slits of the outer periphery side slit inside.