PURPOSE: To obtain an optimum threshold level with simple constitution for a synchronism deciding device for receiver of spectrum diffusion communication, by changing the threshold level in response to the level of the output signal of an integrator.

CONSTITUTION: The correlative output of the reception signal supplied to a terminal 1 and the pseudo noise code generated at the inside of a receiver and supplied to a terminal 2 is integrated every prescribed period. Then the final integration value is compared with the threshold level set at first. A synchronism acquisition signal is produced from a comparator 9 when the final integration value is larger than the threshold level. An up/down counter 10 supplies the counted-up signal to a delay circuit 11 every time the said acquisition signal is produced. Thus the threshold level is raised up to the prescribed value.