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To shield the environmental magnetic noises, magnetically shielded rooms (MSRs) are used for biomagnetic measurements, etc. The sources of environmental noises are placed far away from the MSR, in such way that almost uniform magnetic fluxes are applied to the MSR. Therefore, to specify the shielding performance of MSR, the shielding factor SFÂ¿ for the uniform magnetic flux density should be evaluated. To evaluate SFÂ¿, in a construction site, the magnetic noise is applied by coils. However, SF's depend on the size and position of the coil. Therefore, to accomplish a standardization of the method for evaluating SFÂ¿, accurately, by coils, the method for estimating SFÂ¿'s by smaller coil placed near MSR is being investigated in JEITA (Japan Electronics and Information Technology Industries Association). In JEITA, we have already proposed the method of estimating SFÂ¿'s by the extrapolation using simple numerical expressions representing the relations between the SF and the size, position of coil. The proposed method was verified by the actual single-layered MSR using coils placed on double sides. In this paper, the method of estimating SFÂ¿ of the double-layered MSR, using coils placed only on one side of MSR and taking account of door effect, is investigated using 3D nonlinear magnetic field analysis and measurements. By using the analysis, it is shown that the single coil should be set in front of the door. Moreover, SFÂ¿'s estimated by using the measured data with the optimal coil setting can almost represent the tendencies of the true SFÂ¿.