High reliability assembly techniques for miniaturized in-line pressure sensor with diaphragm using a thin strain sensor have been developed to monitor the hydraulic pressure in the pipetting such as dispensing devices. The pressure sensor with a thin strain sensor as a diaphragm has two problems, (i) water leakage and (ii) sensitivity reduction by adding hydraulic pressure. These problems were clarified to generate tensile stress in the adhesive (Ag paste) by adding hydraulic pressure and to peel strain sensor from SUS housing. To overcome these problems, the stress in Ag paste was needed to keep compressive stress even when adding pressure. In this paper, a pressing mechanism is newly proposed to make compressive stress in Ag paste and assemble on the pressure sensor. First, to press the strain sensor to the SUS housing, a cap was bonded on the strain sensor with wafer-level-packaging (WLP) technique. Next, a pressing mechanism was designed to assemble on the miniaturized pressure sensor. The pressing mechanism was composed of a cover, two coil springs, and a spring base with pressing point. The pressing mechanism generates compressive stress to Ag paste even under high hydraulic pressure. The pressure sensor which implements the pressing mechanism has high pressure resistance and high reliability under the repetitive hydraulic pressure of 10 million times, without water leakage and sensitivity decrease.