A boom slewing angle detection device for a mobile crane to detect a slewing angle of a crane boom from a slewing storage position thereof, the boom slewing angle detection device being characterized by comprising: a slewing angle detecting gear coaxially attached to a shaft end of a pinion gear so as to integrally rotate therewith, the pinion gear being an output element of a slewing reduction gear at a side of a lower travelling body of a mobile crane , the slewing reduction gear transmitting a slewing force to a ring gear attached to a side of the crane boom ; a slewing-angle-detecting proximity sensor to detect an amount of rotation of the slewing angle detecting gear ; a slewing storage position sensor to detect the slewing storage position of the crane boom ; and a controller provided with: a slewing amount calculation part to calculate a slewing amount of the crane boom based on the amount of rotation detected by the slewing-angle-detecting proximity sensor ; a slewing angle calculation part to calculate the slewing angle based on the slewing amount; and a slewing-angle zero-point adjustment part to reset the slewing angle to be zero degree when the slewing storage position sensor detects the slewing storage position, wherein the slewing-angle-detecting proximity sensor and the slewing storage position sensor are adjacently disposed at an outside of the ring gear along a direction of an outer peripheral surface of the ring gear in a manner sandwiching the slewing angle detecting gear ; and the slewing angle detecting gear , the slewing-angle-detecting proximity sensor and the slewing storage position sensor are covered by a dust cover attached to the lower travelling body . a slewing angle detecting gear coaxially attached to a shaft end of a pinion gear so as to integrally rotate therewith, the pinion gear being an output element of a slewing reduction gear at a side of a lower travelling body of a mobile crane , the slewing reduction gear transmitting a slewing force to a ring gear attached to a side of the crane boom ; a slewing-angle-detecting proximity sensor to detect an amount of rotation of the slewing angle detecting gear ; a slewing storage position sensor to detect the slewing storage position of the crane boom ; and a controller provided with: a slewing amount calculation part to calculate a slewing amount of the crane boom based on the amount of rotation detected by the slewing-angle-detecting proximity sensor ; a slewing angle calculation part to calculate the slewing angle based on the slewing amount; and a slewing-angle zero-point adjustment part to reset the slewing angle to be zero degree when the slewing storage position sensor detects the slewing storage position, wherein the slewing-angle-detecting proximity sensor and the slewing storage position sensor are adjacently disposed at an outside of the ring gear along a direction of an outer peripheral surface of the ring gear in a manner sandwiching the slewing angle detecting gear ; and the slewing angle detecting gear , the slewing-angle-detecting proximity sensor and the slewing storage position sensor are covered by a dust cover attached to the lower travelling body . <EOS>