To provide a trend graph display device with improved usability which enables the state, etc., of data to speedily be grasped.

The trend graph display device having a display controller 1, a display part 3, and an input part 5, is equipped, in the display controller 1 with a graph information storage part 19, a display image selection part 11, a total display control part 16, a partial trend graph display control part 17, a subscreen display control part 18, and a main-subscreen interlockedly processing part 15 which gives instructions so that a part other than a subscreen at the display part 3 is displayed as a main screen, and when display alteration occurs in the main screen, the corresponding part of the subscreen is updated and displayed, and the main screen is updated and displayed interlocked with display alteration from the subscreen.