[en] We have developed a dilatometric measuring system for thermal expansion and magnetostriction, those are more singular than specific heat in approaching to a quantum critical point. With decreasing temperature, thermal expansion becomes small in proportional to the square of temperature, thus, high sensitivity and reproducibility are necessary for the dilatometric measurements in millikelvin temperatures. Our dilatometer composed of the sample and the reference capacitor provides the extremely high resolution of ΔL/L ∼ 10"−"1"0 using the ratio-transformer-based capacitance bridge. The dilatometer was installed on the "3He-"4He dilution refrigerator with the 9 T superconducting magnet, and temperature was measured by the "3He melting curve thermometer. We have measured thermal expansion and magnetostriction of the typical heavy fermion compound CeRu_2Si_2 along a-axis at temperature down to 10 mK in magnetic fields up to 9 T