In our previous research, a method for evaluating the corrosion behaviour of unpainted steel plates using an Fe/Ag atmospheric corrosion monitor (ACM) sensor was proposed. In Japan, the atmospheric environment needs to be monitored over all the four seasons in a year to monitor the effect of climate change. In high corrosion environments, Fe/Ag ACM corrosion sensors deteriorate at an early stage, and consequently they must be frequently replaced. This is very uneconomical and makes such sensors hard to use. To solve this problem, a Zn/Ag ACM sensor which has much longer life than an Fe/Ag ACM sensor has been attracting attention. If a Zn/Ag ACM sensor can be used as an alternative tool, it will be possible to make monitoring systems more economical. Thus, to propose a method for evaluating the corrosivity of the atmospheric environment with a Zn/Ag ACM sensor, the relationship between the output of an Fe/Ag ACM sensor and of a Zn/Ag ACM sensor was investigated and the difference was clarified.