An oil deterioration detector comprising: a sensor configured to measure electrical characteristics of an oil; a main body having a straight rod shape thereby defining an axial direction and an radial direction, wherein the main body is configured to be attached to the sensor, where the sensor is attached in a posture facing in the radial direction at one end of the main body and configured to detect a degree of deterioration of oil from the electrical characteristics, and a sensor cover (2, 2A, 2B, 2C, 2D) detachably attached to the one end of the main body, the sensor cover (2, 2A, 2B, 2C, 2D) comprising: a base part with cylindrical shape arranged on a main body side, a tip head part arranged on a tip side of the sensor, and a connection part for connecting the base part and the tip head part in the axial direction, wherein the connection part comprises at least one opening (24, 25, 24B, 25B, 25C, 24D, 25D) and at least one connecting pillar (231, 232), wherein the tip head part has an arcuate shape or an annular shape being equal to or greater in size in the radial direction; wherein the sensor cover (2, 2A, 2B, 2C, 2D) is configured to be rotated by the user between an attachment reference posture and a cleaning posture, when the sensor cover (2, 2A, 2B, 2C, 2D) is attached to the main body, the tip head part is configured to be arranged to protrude at the tip side of the sensor in the attachment reference posture, characterized in that the at least one opening (24, 25, 24B, 25B, 25C, 24D, 25D) has a width (D24, D25) equal to or greater than a width of the sensor; the at least one connecting pillar (231, 232) has a width (D231, 231A, 231B) equal to or greater than the width of the sensor; wherein the at least one connecting pillar (231, 232) is facing and covering the sensor in the attachment reference posture; and wherein the at least one opening (24, 25, 24B, 25B, 25C, 24D, 25D) is facing and covering the sensor in the cleaning posture. a sensor configured to measure electrical characteristics of an oil; a main body having a straight rod shape thereby defining an axial direction and an radial direction, wherein the main body is configured to be attached to the sensor, where the sensor is attached in a posture facing in the radial direction at one end of the main body and configured to detect a degree of deterioration of oil from the electrical characteristics, and a sensor cover (2, 2A, 2B, 2C, 2D) detachably attached to the one end of the main body, the sensor cover (2, 2A, 2B, 2C, 2D) comprising: a base part with cylindrical shape arranged on a main body side, a tip head part arranged on a tip side of the sensor, and a connection part for connecting the base part and the tip head part in the axial direction, wherein the connection part comprises at least one opening (24, 25, 24B, 25B, 25C, 24D, 25D) and at least one connecting pillar (231, 232), wherein the tip head part has an arcuate shape or an annular shape being equal to or greater in size in the radial direction; wherein the sensor cover (2, 2A, 2B, 2C, 2D) is configured to be rotated by the user between an attachment reference posture and a cleaning posture, when the sensor cover (2, 2A, 2B, 2C, 2D) is attached to the main body, the tip head part is configured to be arranged to protrude at the tip side of the sensor in the attachment reference posture, characterized in that the at least one opening (24, 25, 24B, 25B, 25C, 24D, 25D) has a width (D24, D25) equal to or greater than a width of the sensor; the at least one connecting pillar (231, 232) has a width (D231, 231A, 231B) equal to or greater than the width of the sensor; wherein the at least one connecting pillar (231, 232) is facing and covering the sensor in the attachment reference posture; and wherein the at least one opening (24, 25, 24B, 25B, 25C, 24D, 25D) is facing and covering the sensor in the cleaning posture.