A multi-feed detection device which judges whether a sheet-shaped-object, which has a pair of sheet surfaces and is conveyed along a conveyance path toward a conveying direction set previously, is the one sheet-shaped object or the plurality of the overlapped sheet-shaped objects, comprising: a base member fixed to a detection position which is in a middle part of the conveyance path and is opposite to the sheet-shaped object conveyed along the conveyance path; an arm member which has a contact part contacting one of the sheet surfaces of the sheet-shaped object conveyed along the conveyance path, is supported rotatably by the base member, and is applied thereto with biasing force so as to be rotated for making the contact part approach the conveyance path, wherein the contact part contacts the one of the sheet surfaces of the sheet-shaped object conveyed along the conveyance path so that the arm member is rotated so as to make the contact part approach or be separated from the conveyance path oppositely to the biasing force; a magnet fixed to the arm member and moved following the rotation of the arm member; a magnetic sensor which is fixed to a position opposite to the magnet in the base member and outputs an electric signal corresponding to a magnetic field changed by the movement of the magnet; a differentiation circuit which is connected to the magnetic sensor and outputs a differential electric signal corresponding to a differential value of the electric signal outputted by the magnetic sensor; and an integration circuit which is connected to the differentiation circuit and outputs an integral electric signal corresponding to an integral value of the differential electric signal outputted by the differentiation circuit, characterized in that the arm member is formed by a circular member which is bent fan-like when viewed in side, a center of the fan-like shape is rotatably supported by the base member, one of the ends of an arc part of the fan-like shape is formed as the contact part, and the magnet is fixed to the other end of the arc part of the fan-like shape. a base member fixed to a detection position which is in a middle part of the conveyance path and is opposite to the sheet-shaped object conveyed along the conveyance path; an arm member which has a contact part contacting one of the sheet surfaces of the sheet-shaped object conveyed along the conveyance path, is supported rotatably by the base member, and is applied thereto with biasing force so as to be rotated for making the contact part approach the conveyance path, wherein the contact part contacts the one of the sheet surfaces of the sheet-shaped object conveyed along the conveyance path so that the arm member is rotated so as to make the contact part approach or be separated from the conveyance path oppositely to the biasing force; a magnet fixed to the arm member and moved following the rotation of the arm member; a magnetic sensor which is fixed to a position opposite to the magnet in the base member and outputs an electric signal corresponding to a magnetic field changed by the movement of the magnet; a differentiation circuit which is connected to the magnetic sensor and outputs a differential electric signal corresponding to a differential value of the electric signal outputted by the magnetic sensor; and an integration circuit which is connected to the differentiation circuit and outputs an integral electric signal corresponding to an integral value of the differential electric signal outputted by the differentiation circuit,