A simple apparatus for measuring the magnetism of magnetotactic bacteria was developed with a common laboratory spectrophotometer, which was based on measuring the change in light scattering resulting from cell alignment in a magnetic field. A multiple coils were built around the cuvette holder of the spectrophotometer to compensate geomagnetic field and to generate two mutually perpendicular magnetic fields. In addition, we defined a novel magnetism parameter, R(mag), by modifying the definition of C(mag) to a normalized parameter with the culture absorbance obtained without application of magnetic field. The number of magnetosomes in each cell was determined by transmission electron microscopy to assess the relationship between the two magnetism parameters and the distribution of magnetosomes in the cells. We found that both R(mag) and C(mag) were linearly correlated rather with the percentage of magnetosome-containing bacteria than with the average magnetosome numbers, and R(mag) exhibited a better linearity than C(mag) with respect to the percentage of magnetosome-containing bacteria.