This paper describes new method for selecting optimal field points in Inverse-Numerical Acoustic analysis (INA), and an application to construction of sound source model for diesel engine. INA is a method that identifies surface vibration of the sound source by using acoustic transfer functions and actual sound pressures which are measured at field points located near the sound source. In the INA, for measuring sound pressures, it is necessary to determine the arrangement of field points. The increase of field points leads to longer test and analysis time. Therefore, guidelines for deciding field point arrangement are needed to conduct INA efficiently. The authors focused on the standard deviations of distance between sound source elements and field points, and proposed new guideline for optimal selection of the field points, in a past study. In the past study, the effectiveness of this guideline was verified using a simple plate model. In this study, we used new guideline for optimal selection of the field points, and constructed the sound source model of diesel engine which installed on an agricultural machine. In the measurement of sound pressure at field points, the microphone spacing and distance were decided by using new guideline. Moreover, the number of the field point was changed by analysis frequency. It was confirmed that it is possible to construct sound source model efficiently by using new guidelines. In addition, the surrounding sound pressure level of engine was predicted using the sound source model, and the accuracy of sound source model was confirmed.