A water-depth measuring apparatus for a double bearing type fishing reel including a reel body in which a spool carrying a fishing line is rotatably supported. At least one element is provided on the spool for rotation therewith. A case which is separate from the reel body is removably mountable on the reel body, and contains a detector for detecting the rotating speed of the element and generating a corresponding signal, a computer for receiving the signal from the detector and calculating the cast length of the fishing line, a display device for displaying the cast length, a keyboard, a battery and a buzzer. The keyboard carries a power supply ON key, a power supply OFF key, a fishing line size set key, a memory set key and a buzzer OFF key. The detector faces the element and the buzzer sounds when the cast length of the fishing line has reached a certain level.