A new type of the long-period seismometer, called the gyro-transducer, has previously been made^{3), 4)}. The observation system for measuring horizontal seismic motion has the distinguishing point which is its potential for measuring two-components as a vector with the gyro-transducer. However, dynamical characteristics about a roll-axis and a pitch-axis are usually coupled to each other. If this cross-coupling is undesirable, it must be minimized by modifying the structure of the system. On a microcomputer data processing system, this is achieved by 1) identifying parameters and 2) decoupling the interaction into isolated parts that can be described by a simple dynamic relation. Furthermore, a new type of the recording system for the continuous observation of earthquakes is developed.