PURPOSE: To provide a monitor device which performs comparison very quickly at the time of acquiring a sample.

CONSTITUTION: A certain number of upper bits 8 of each sample (12 bits) supplied by an analog-digital converter 3 are used as an address AD1 of a comparison area 6a of a RAM 6 related to a microprocessor 5 of the device. The results of comparison between this sample and a prescribed number of thresholds are stored in this comparison area 6a. Comparison results (4 bits) of the sample (12 bits) are stored in a storage area 6b of the RAM 6 so as to be processed in the microprocessor 5 later. The sample and preliminarily stored results of comparison between this sample and thresholds are simultaneously obtained to shorten the time required for data acquisition in the microprocessor 5.