To provide a magnetic body quantity detector for accurately detecting a magnetic body quantity included in a magnetic ink on paper sheets.

This magnetic body quantity detector is provided with first and second magnetoresistive elements 4 and 5 disposed so as not to overlap with each other for detecting magneto-resistance of a magnetic body 8 formed on a conveyed medium 7 under detection, and a permanent magnet 6 for giving vertical and horizontal magnetic force to the first and second magnetoresistive elements 4 and 5, respectively. A DC voltage is impressed on the first and second magnetoresistive elements 4 and 5, and a difference between their output signals is used to obtain the body quantity of the magnetic body 8 formed in the medium 7 under detection.