PURPOSE: To stand still and display even in case of a repeated phenomenon at a low speed, by reading out repeatedly a data from a storing circuit independently from rewriting of the data, and also executing the rewriting of the data from the same phase every time.

CONSTITUTION: When writing a data to a storing circuit M, resetting of a write counter CW is released, it is executed as designating an address to the storing circuit M through a controlling circuit CT, and data of 1W(n) addresses in the circuit M are rewritten successively. As for the read-out operation, the address designation is executed to the storing circuit M by contents of a counter CR, and the data of 1W(n) addresses are read out successively. This data is latched by a latching circuit L, and thereafter, is D/A-converted, and the vertical deflection is executed. Accordingly, a waveform before rewriting the data and a waveform before the rewritng stand still and are displayed on the picture. Accordingly, even in case of a repeated waveform at a low speed, it stand still and can be displayed, and also even if an input waveform is varied, it can be displayed successively.