The cross suspension pendulum system has been alreadly adopted as key mechanismm in the detector of TEM tiltmeter. As the pendulum system has special character that magnifies the tilting of its support, it will be applicable not only to tiltmeters but to other instruments which detect small amount of displacement. In this paper, the various natures of the cross suspension pendulum, such as its equilibrium position, sensitivity to tilting, period of the pendlum and their relations are theoretically treated. In these relations, it is noticeable that the sensitivity is approximately propotional to the square of the period of the pendulum on condition of normal use.