A sheet processing apparatus comprising: a housing case; a contactless type magnetic sensor array unit provided in the housing, comprising: a detection magnetic sensor part configured to detect magnetism of a sheet when the sheet is arranged inside the housing so as to oppose the detection magnetic sensor part; and a correction magnetic sensor part (300L, 300R) which does not oppose the sheet when the sheet is arranged inside the housing so as to oppose the detection magnetic sensor part, the correction magnetic sensor part (300L, 300R) being configured to detect a magnetic field inside the housing, the magnetic field being near the detection magnetic sensor part and arising from environmental sources when the sheet is not inside the housing; 3D coils (CxL, CxR, CyL, CyR, CzL, CzR, CxS, CzS) provided in, or attached to, the housing case, and near the detection magnetic sensor part; and a cancel device operable to drive the coils (CxL, CxR, CyL, CyR, CzL, CzR, CxS, CzS) to generate a cancellation magnetic field for compensating for an environmental magnetic noise inside the housing case, thereby reducing the environmental magnetic noises included in the electric signals detected by the detection magnetic sensor part, based on a detection result of the correction magnetic sensor part (300L, 300R). a housing case; a contactless type magnetic sensor array unit provided in the housing, comprising: a detection magnetic sensor part configured to detect magnetism of a sheet when the sheet is arranged inside the housing so as to oppose the detection magnetic sensor part; and a correction magnetic sensor part (300L, 300R) which does not oppose the sheet when the sheet is arranged inside the housing so as to oppose the detection magnetic sensor part, the correction magnetic sensor part (300L, 300R) being configured to detect a magnetic field inside the housing, the magnetic field being near the detection magnetic sensor part and arising from environmental sources when the sheet is not inside the housing; a detection magnetic sensor part configured to detect magnetism of a sheet when the sheet is arranged inside the housing so as to oppose the detection magnetic sensor part; and a correction magnetic sensor part (300L, 300R) which does not oppose the sheet when the sheet is arranged inside the housing so as to oppose the detection magnetic sensor part, the correction magnetic sensor part (300L, 300R) being configured to detect a magnetic field inside the housing, the magnetic field being near the detection magnetic sensor part and arising from environmental sources when the sheet is not inside the housing; 3D coils (CxL, CxR, CyL, CyR, CzL, CzR, CxS, CzS) provided in, or attached to, the housing case, and near the detection magnetic sensor part; and a cancel device operable to drive the coils (CxL, CxR, CyL, CyR, CzL, CzR, CxS, CzS) to generate a cancellation magnetic field for compensating for an environmental magnetic noise inside the housing case, thereby reducing the environmental magnetic noises included in the electric signals detected by the detection magnetic sensor part, based on a detection result of the correction magnetic sensor part (300L, 300R).