Performance of a gold film mercury detector and its application for detection of ambient mercury vapor are described. Mercury vapor adsorbed on a thin gold film resistor makes a small resistance change which is detected by a Wheatstone bridge constructed with 4 gold film resistors themelves and IC amplifiers out of which a voltage signal corresponding to the amount of mercury is fed to and displayed on an electrical recorder. The detection limit of the equipment is about 30-50 pg. The Hg vapor calibration curves extended from 0.2 to 200 ng in good linearity are reproducible to within 10% at 0.2-2 ng, and to within 3-4%, relative standard deviation at 4-40 rg. Examinations were made on the selection of a carrier gas which feed Hg vapor into the epuipment, and collection capability of gold wool collector used to pre-accumlate ambient Hg by adsorption and making an impulse Hg signal by desorption all of these factors are important in practice. This gold film resistor method and an atomic absorpion method were compared. The correlation between these two methods showes a good linearity from 2.6 to over 13 ng. Ambient Hg in a room is collected on the gold wool air sampler and detected by this method, and the amount of Hg recorded increased increasing time interval of cellection. The working collection time to obtain a reasonable data of mercury by the air sampler is about 30-60 min, at the air flow rate of 1 liter/min.