Abstract: A magnetoresistive element that has a magnetic material made of an alloy having a stable bcc structure containing Co as a main component, has an excellent tunnel magnetoresistive ratio, and can be put into practical use by mass production, and a magnetic storage device using the magnetoresistive element are provided. The magnetoresistive element includes a first magnetic layer whose magnetization direction is substantially fixed, a second magnetic layer whose magnetization direction is changeable, and a non-magnetic layer arranged between the first magnetic layer and the second magnetic layer. The first magnetic layer and/or the second magnetic layer has an alloy having a bcc structure containing Co as a main component and Co and Mn.