Patent ID: 7388809

Claim:
A fish finder comprising: a transducer that repeatedly transmits ultrasonic pulse signals downward into water and receives echo signals from the water; an A/D converter that converts the signal received by the transducer into a digital signal; a memory that stores the signal outputted from the A/D converter; an interference detecting unit that detects an occurrence of interference on the basis of a reception signal belonging to a predetermined depth range among reception signals stored in the memory; and an interference removing unit that when the interference detecting unit detects an occurrence of interference, outputs a signal obtained by applying predetermined interference removal processing to the reception signal, wherein the interference detecting unit calculates, for each identical depth, a difference value between intensity of a reception signal received at a time and intensity of a reception signal received at a previous time and, when a value obtained by adding up difference values over the depth range is larger than a predetermined threshold, detects an occurrence of interference.