An ion detector 1A for detecting positive ions is provided with a chamber 2 having an ion entrance 3 which allows positive ions to enter, a conversion dynode 9 which is disposed in the chamber 2 and to which a negative potential is applied, and an avalanche photodiode 30 that is disposed in the chamber 2 and has an electron incident surface 30a which is opposed to the conversion dynode 9 and also into which secondary electrons emitted from the conversion dynode 9 are made incident. The electron incident surface 30a is located closer to the conversion dynode 9 than a positioning part 14 which supports the avalanche photodiode 30 in the grounded chamber 2.