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We have developed a “sensor particle”, which is a Lagrangian sensor system for real-time measurement of the force acting on a particle moving freely in fluid flow. The sensor particle is composed of an acceleration sensor, magnetometer, microcomputer, wireless module and cells. The built-in acceleration sensor detects the acceleration of particle at each instant of time. However, the signals contain both dynamic and static (gravitational) accelerations. Therefore we applied the external magnetic field oriented in the gravitational direction in order to identify the attitude of the sensor particle and to remove the gravitational acceleration from the measured signals. Consequently, we can obtain the force acting on a particle moving in fluid at each time through the equation of the motion. We have applied our system to oscillatory liquid flow with particle assemblage to obtain the basic knowledge on unsteady fluid force in a multi-particle system.