To make it easy to grasp the operation state of a work vehicle and to make the management easy by storing a decided operation time as a time count and displaying the operation time as a time stamp time on a display device.

When a display device is connected to a vehicle unit 31 mounted on a work vehicle, an operation start time, an operation end time, an operation time and an accumulative operation time are calculated based on the data of an operation time of the work vehicle which is stored by the unit 31 and shown on the display screen. A sampling signal inputted to an operation processing circuit 33 counts time count as a counting signal in every counting signal. The circuit 33 decides the change of a signal S1 and decides whether or not a set logical condition is satisfied, when the logical condition is satisfied, a decision output signal is outputted and the change of the signal and the time count are recorded in a storage circuit 34. The operation time stored in this way is shown as a time stamp time on the display device.