Dilution of a sample occurring in a sample dispensing operation is a problem in terms of analysis accuracy. Although the dilution amount is alleviated to some extent by a technique of sucking the sample excessively, the problem may emerge along with a requirement for reducing the sample dispensing amount, an increase of sucking and discharging speed in response to improvement of a processing capability, and an increase of the number of items to be analyzed simultaneously. Provided is an automatic analyzer including a mechanism monitoring conductivity of system water filled in a nozzle and an electro-physical amount such as voltage and capacitance of a probe to enable detection of mixture of the sample and a reagent into the system water and dilution of the sample and the reagent.