PURPOSE: To control the insertion of macroblocks by a simple circuit by selecting a number (n) which is mutually prime with the number of macroblocks in a frame and performing forcible in-frame encoding at intervals of (n) macroblocks.

CONSTITUTION: When a macroblock(MB) start signal is inputted, the MB start signal increments the count value of an MB counter 101 by one unless a reset input is active. A 130 detecting circuit 102 inputs the count value of the counter 101 and outputs a forcible in-frame encoding signal which becomes active only when the count value is equal to 130. This request signal is outputted to the outside and also inputted to an AND gate 103. This request signal is inputted to the other input of the gate 103 and only when the MB start signal is active, the gate 103 is turned ON to send the signal to the reset input of the counter 101 through an OR gate 104, thereby resetting the count value output of the counter 101 to 0.