This axial deviation estimating device estimates an axial deviation angle of a radar device mounted on a mobile body, and includes an acquiring unit, an extracting unit, a device-system coordinates unit, and an estimating unit. The estimating unit estimates an axial deviation angle using a relational expression. The relational expression is an expression that holds between at least one unknown parameter, which includes an axial deviation angle of a coordinate axis of the radar device about a target axis which is at least one of a horizontal axis and a traveling direction axis constituting the coordinate axes of the mobile body, and at least one element included in the device-system coordinates of a road surface reflection point.