A boom attitude detection apparatus that calculates an attitude of a boom of a crane, the boom attitude detection apparatus comprising: a GPS device having a base station GPS antenna provided on a slewing base and a mobile station GPS antenna provided on a leading end of the boom, the GPS device performing positioning of a position of a leading end of the boom based on received data from the base station GPS antenna and the mobile station GPS antenna; an attitude detector (31, 32) that detects an attitude of the boom; characterized by a load detector that detects a load applied to the boom; and a calculation device that receives a positioning value of the GPS device and detection values of the attitude detector (31, 32) and the load detector, wherein the calculation device stores therein a correction table in which a correction value per attitude of the boom and load applied to the boom is stored, and in a case where positioning reliability of the GPS device is high, the calculation device calculates a correction value for correcting a detection value of the attitude detector (31, 32) to an attitude of the boom to be calculated based on a positioning value of the GPS device, and updates, by the calculated correction value, correction values within the correction table corresponding to detection values of the attitude detector (31, 32) and the load detector, and in a case where positioning reliability of the GPS device is low, the calculation device obtains correction values corresponding to detection values of the attitude detector (31, 32) and the load detector from the correction table, and corrects a detection value of the attitude detector (31, 32) by the obtained correction values to calculate an attitude of the boom. a GPS device having a base station GPS antenna provided on a slewing base and a mobile station GPS antenna provided on a leading end of the boom, the GPS device performing positioning of a position of a leading end of the boom based on received data from the base station GPS antenna and the mobile station GPS antenna; an attitude detector (31, 32) that detects an attitude of the boom; characterized by a load detector that detects a load applied to the boom; and a calculation device that receives a positioning value of the GPS device and detection values of the attitude detector (31, 32) and the load detector, wherein the calculation device stores therein a correction table in which a correction value per attitude of the boom and load applied to the boom is stored, and in a case where positioning reliability of the GPS device is high, the calculation device calculates a correction value for correcting a detection value of the attitude detector (31, 32) to an attitude of the boom to be calculated based on a positioning value of the GPS device, and updates, by the calculated correction value, correction values within the correction table corresponding to detection values of the attitude detector (31, 32) and the load detector, and in a case where positioning reliability of the GPS device is low, the calculation device obtains correction values corresponding to detection values of the attitude detector (31, 32) and the load detector from the correction table, and corrects a detection value of the attitude detector (31, 32) by the obtained correction values to calculate an attitude of the boom.