PURPOSE: To determine rapidly urea in the whole blood and etc., by detecting the change of electroconductivity resulting from a reaction of the urea with a absorption liquid through a gas-permeable membrane, after the urea in a sample to be tested is converted to NH4+ by a enzyme reaction and then to NH3.

CONSTITUTION: A definite amount of a sample to be tested is injected from a sample injector 5 into the 3rd pass way 10c, and transferred with a carrier solution 2c to a immobilized enzyme 6. Subsequentry, urea is subjected to enzyme reaction to produce NH4+ and CO3-. These are transferred with the carrier solution to a junction A, then mixed with a mixture solution 2b, further subjected to reaction. NH3 thus produced is transferred again with the carrier solution to the 2nd chamber 9b in a flow cell 9, then transmitted through a membrane 9 to the 1st chamber 9c, and reacted with an absorption solution 2a to be absorbed as NH4NO3. The change of electroconductivity before and after the reaction is measured by a electrode 7 and an electroconductivity meter, and the amount of urea in the sample is determined by the result.