Abstract: Data are sampled manually using a electromagnetic induction type underground metal pipe detectror. After that, an empirical formula for a metal pipe s second field distribution function is obtained by fitting the data. On the other band, the depth of underground metal pipe can be detected by use of the detector. Then, the pipe s diameter can be calculated by use of the half width of the distribution func-tion s curve and the depth. The error of the result is 10% of the pipe s diameter.