PURPOSE: To provide a speech which gives no feeling of physical disorder by detecting the head and tail of the speech as to the speech detector which detects a speech included in a pulse-code modulated speech.

CONSTITUTION: After the DC component of the pulse code modulated speech which is inputted is passed through a high-pass filter 1 which cuts of the DC component, the output of the high-pass filter 1 is applied to a zero-cross quantity counter 2, an input electric power averaging part 3, and a predicted gain variation rate part 4 to calculate levels, frame by frame, and the calculation result is applied to a decision part 5 to detect whether or not there is the speech. This speech detector is provided internally with a sample power monitor part 6 which monitors the speech not in frame units, but in sample units after a no-sound decision made once, thereby detecting the head and tail of the pulse code modulated speech.