An adjusting screw structure of an oil immersed solenoid in which: a movable core is provided in a space of a case so as to be movable in an axial direction; the movable core is biased by an adjusting spring in a direction in which the movable core is suctioned by a fixed magnetic pole portion or in an opposite direction thereof; and an adjusting screw capable of adjusting biasing force of the adjusting spring is included, the adjusting screw structure comprising an air-bleeding plug configured to be detachably attached to and seal an opening end portion of the case, the opening end portion being communicated with the space of the case, wherein: the air-bleeding plug includes an internal screw portion with which the adjusting screw threadedly engages and a detachable mechanism configured to be able to attach the air-bleeding plug to the opening end portion of the case such that the air-bleeding plug is located at a predetermined attachment position in a movement direction of the adjusting screw; and the detachable mechanism includes a detachable screw portion configured to attach the air-bleeding plug to the opening end portion of the case such that the air-bleeding plug threadedly engages with the opening end portion, an annular sealing portion configured to be attached to an annular groove formed on an outer peripheral surface of the air-bleeding plug; a sealing inclined surface configured to be formed in an annular shape on an inner peripheral surface of the opening end portion of the case, having an inner diameter increasing in size toward the opening, and tightly contacting the annular sealing portion when the air-bleeding plug is attached at an attachment position of the opening end portion, and an air-bleeding hole configured to be formed on the air-bleeding plug and allow the space of the case and an outside of the oil immersed solenoid to be communicated with each other when the air-bleeding plug is loosened. the air-bleeding plug includes an internal screw portion with which the adjusting screw threadedly engages and a detachable mechanism configured to be able to attach the air-bleeding plug to the opening end portion of the case such that the air-bleeding plug is located at a predetermined attachment position in a movement direction of the adjusting screw; and the detachable mechanism includes a detachable screw portion configured to attach the air-bleeding plug to the opening end portion of the case such that the air-bleeding plug threadedly engages with the opening end portion, an annular sealing portion configured to be attached to an annular groove formed on an outer peripheral surface of the air-bleeding plug; a sealing inclined surface configured to be formed in an annular shape on an inner peripheral surface of the opening end portion of the case, having an inner diameter increasing in size toward the opening, and tightly contacting the annular sealing portion when the air-bleeding plug is attached at an attachment position of the opening end portion, and an air-bleeding hole configured to be formed on the air-bleeding plug and allow the space of the case and an outside of the oil immersed solenoid to be communicated with each other when the air-bleeding plug is loosened. a detachable screw portion configured to attach the air-bleeding plug to the opening end portion of the case such that the air-bleeding plug threadedly engages with the opening end portion, an annular sealing portion configured to be attached to an annular groove formed on an outer peripheral surface of the air-bleeding plug; a sealing inclined surface configured to be formed in an annular shape on an inner peripheral surface of the opening end portion of the case, having an inner diameter increasing in size toward the opening, and tightly contacting the annular sealing portion when the air-bleeding plug is attached at an attachment position of the opening end portion, and an air-bleeding hole configured to be formed on the air-bleeding plug and allow the space of the case and an outside of the oil immersed solenoid to be communicated with each other when the air-bleeding plug is loosened.