The invention provides a speaker capable of accurately detecting an electrostatic capacity formed between a central magnetic pole and a voice coil bobbin. A first and a second electrodes (18 and 19) are arranged on an inner peripheral surface (15b) at a constant interval along the axis of the central magnetic pole, and a step par (5e) which increases the area of opposition between one electrode and a side peripheral surface (5c) and decreases the area of opposition between the other electrode and the side peripheral surface (5c) by the increment when the voice coil bobbin (4) operates is formed on the side peripheral surface (5c), and the speaker (1) is provided with a subtracter (31) which subtracts a second electric signal obtained by the other electrode from a first electric signal obtained by the one electrode and outputs a third electric signal corresponding to the subtraction result. |