The correlation of acetone concentration in blood with that in the air expired by patients suffered from diabetes mellitus was revealed. This correlation may become the basic one for invasive acetone determination in patient's blood via analysis of the air expired. The factors which interfere with acetone determination are: (i) smoking just before the analysis and (ii) the alcohol presence in patients' blood at the moment of analysis. Based on the sensor developed, the apparatus was designed for ketoacidosis diagnosis by means of acetone determination in expired air accounting for the peculiarities of human breath-out dynamics. Acetone determination by means of the apparatus designed is an express one. It is much more simple, does not involve special carrier gas and is less expensive than GLC or photometry analysis.