A secondary electron multiplier includes: a conversion dynode for emitting a secondary electron in response to an incident ion; a plurality of dynodes configured to have multi-stages from second to final stages for receiving the secondary electron; and a first voltage applying device for applying a first negative voltage to the conversion dynode and sequentially dividing the first negative voltage to apply to each of the second-stage and subsequent dynodes, wherein the secondary electron multiplier is configured to sequentially multiply the emitted secondary electron by the second-stage and subsequent dynodes. In the secondary electron multiplier, any of the second-stage and subsequent dynodes have a second voltage applying device for applying a second negative voltage. The secondary electron multiplier has an improved ion detection efficiency without a large reduction of a usable period thereof, thereby enhancing the sensitivity of a mass spectrometer.