Increased pressure, which has been strongly desired for diesel fuel injection equipment, has many technological challenges. One of the biggest ones is the increase in temperature caused by leakage fuel. The investigation of constant leakage from sliding portions inside injectors reveals that the deformed structure due to the increased pressure substantially affects leakage quantity. It also indicates that the leakage quantity which is in proportion to pressure by the base equation, is not, and takes higher degree than 1. Based on the results, the estimated leakage quantity has approximately threefold increase when pressure increases from 200 MPa to 300 MPa.