To provide a hydraulic motor swinging unit having an automatic oil level detecting function of immediately giving a warning to an operator for a construction machine in the case of deviating from a predetermined oil level without the operator needing to directly handle a level gauge.

A discriminating part 102 is supplied with logical value signals T1, T2 from an oil level sensor S having two floats. When any one of A, B, C is determined as the determined result of a determining part, a CPU 112 transfers contents recorded in a memory part corresponding to one of the A, B, C, to a display part 106 and/or a voice output part 108. The operator thereby confirms immediately with a voice and/or a display in an operating room whether the oil level is within a normal range, and whether the oil level is high or low in the case of exceeding the normal range.