Disclosed herein is a method of manufacturing an inertial sensor. The method includes: (A) preparing a base substrate; (B) forming a depressed first concave part in one surface of the base substrate; (C) forming a mass body in the first concave part by filling a metal or a combination of a metal and a polymer (or a polymer matrix composite) therein; and (D) forming a depressed second concave part in one surface of the base substrate at an outer side of the mass body and forming a flexible part on an upper portion of the second concave part in the base substrate. The mass body formed of the metal or the combination of the metal and the polymer (or the polymer matrix composite) has high density, thereby making it possible to improve sensitivity of the inertial sensor.