PURPOSE: To display a temp. distribution image on the basis of a reference temp. being the temp. at the desired point in the temp. distribution image indicated by a position indicating means such as a light pen, by taking the temp. data of a picture element read from a memory means on the basis of an address signal.

CONSTITUTION: The picture element data stored in memory 12 is read along the data of the character information pattern stored in character display memory 13 and both data are sent to a cathode ray tube display apparatus 11 used is monitoring through a D-A converter 15 and a television signal converter circuit 16 while added by an adder 14. Therefore, a temp. distribution image Z and informations related to said image Z, for example, valves of CT (a center temp.) and TW (a temp. range) are displayed in an overlapped state by the picture of a display apparatus. A synchronous signal generating circuit 17 generates reference television synchronous signals H. V to be supplied to the television signal converter circuit 16 while a clock transmitter 18 generates a picture element clock signal for dividing one raster into 256 picture elements on the basis of the synchronous signal H. A counter 19 counts the clock signal to generate a horizontal scanning signal.