Provided is a detector for fishing in order to detect the depth of water, the temperature of water, the distribution of fishes including fishes' migration and/or the length of fishes. The detector includes: a sensor transmitting an ultrasonic signal into the water in a fishing site and receiving a reflective ultrasonic signal reflected from an underwater object; a control unit analyzing/determining the waveform of the reflective ultrasonic signal measured in the sensor to then be indicated on an indication unit including a light-emission indicator, a numeric indicator, and a water depth/temperature indicator; a switching unit having an elevating rod whose lower end contacts an activation switch to turn on/off electric power supplied from a power source and in the upper end of which a fishing line engagement hole is formed in which a fishing line is connected with the fishing line engagement hole and the elevating rod is exposed to the outside to then be elastically pressed to be opened and closed according to the pressure of the elevating rod; and a main body on the upper surface of which a light-emission indicator, a numeric indicator, and a water depth/temperature indicator are formed, and in which the sensor, the control unit, and the switching unit are embedded in a sealed manner with a predetermined structure. The detector further includes a transmitter transmitting a signal representing the analyzed/determined result in the form of a predetermined waveform wirelessly. The wireless signal transmitted from the transmitter in the fishing detector is received and displayed using a predetermined program via a predetermined receiver having a display such as a personal portable communications terminal like a mobile telephone.