A hydraulic cylinder stroke operation diagnosis assisting device includes: movable portions that are sequentially supported by a vehicle body in a rotatable manner; a hydraulic cylinder between the vehicle body and each movable portion or between the movable portions to support the movable portions in a rotatable manner; a stroke sensor in the hydraulic cylinder to measure a stroke length of the hydraulic cylinder; a reset sensor measuring a reference reset point in which a value of the stroke length measured by the stroke sensor is reset; a stroke end detection unit detecting a stroke end position of the hydraulic cylinder; a calibration unit calibrating the measured value of the stroke length when the reference reset point and/or the stroke end position are detected; and a monitor displaying at least the measured value of the stroke length and a calibration state by the calibration unit.