A display device configured to receive data from a higher-level device to display, the display device comprising: a drawing processing unit comprising a plurality of sub-units forming a redundant configuration, each sub-unit comprising: a drawing data generating unit (3, 4), means configured to store a plurality of different variation patterns, and a pattern selector configured to select a variation pattern from the plurality of stored variation patterns; wherein the drawing data generation unit of each sub-unit is configured to control the pattern selector of the respective sub-unit to vary the selected variation pattern by selecting another variation pattern from the plurality of stored variation patterns at regular intervals; wherein the display device further comprises: a display unit and a first comparator configured to compare whether input signals thereof are coincident, wherein each drawing data generating unit includes: a receiver configured to receive said data from the higher-level device; a drawing control unit configured to convert the data received from receiver into drawing data, to retrieve the selected variation pattern at a predetermined timing and to modify the drawing data by interrupting a part of the drawing data and inserting the selected variation pattern, a drawing memory configured to store the modified drawing data and to generate an output signal of the drawing data generating unit regarding the modified drawing data, wherein the display device is configured such that: the plurality of sub-units are redundantly configured to perform identical operations and to select identical variation patterns, the selected variation patterns are input to the drawing control units of the respective drawing data generating units, the output signal of one of the drawing data generating units is transmitted to the display unit, which is configured to display the modified drawing data; the output signals from the drawing data generating units are input to the first comparator to be said input signals thereof, and the first comparator is configured to compare whether the output signals are coincident and to output a comparison result as a detection signal outside. a drawing data generating unit (3, 4), means configured to store a plurality of different variation patterns, and a pattern selector configured to select a variation pattern from the plurality of stored variation patterns; wherein the drawing data generation unit of each sub-unit is configured to control the pattern selector of the respective sub-unit to vary the selected variation pattern by selecting another variation pattern from the plurality of stored variation patterns at regular intervals; a display unit and a first comparator configured to compare whether input signals thereof are coincident, wherein each drawing data generating unit includes: a receiver configured to receive said data from the higher-level device; a drawing control unit configured to convert the data received from receiver into drawing data, to retrieve the selected variation pattern at a predetermined timing and to modify the drawing data by interrupting a part of the drawing data and inserting the selected variation pattern, a drawing memory configured to store the modified drawing data and to generate an output signal of the drawing data generating unit regarding the modified drawing data, a receiver configured to receive said data from the higher-level device; a drawing control unit configured to convert the data received from receiver into drawing data, to retrieve the selected variation pattern at a predetermined timing and to modify the drawing data by interrupting a part of the drawing data and inserting the selected variation pattern, a drawing memory configured to store the modified drawing data and to generate an output signal of the drawing data generating unit regarding the modified drawing data, the plurality of sub-units are redundantly configured to perform identical operations and to select identical variation patterns, the selected variation patterns are input to the drawing control units of the respective drawing data generating units, the output signal of one of the drawing data generating units is transmitted to the display unit, which is configured to display the modified drawing data; the output signals from the drawing data generating units are input to the first comparator to be said input signals thereof, and the first comparator is configured to compare whether the output signals are coincident and to output a comparison result as a detection signal outside.