In a biological sample analyzing apparatus and method, an assay sample is prepared by mixing a reagent with a biological sample which may contain assay material. The photo-mask is a light transmission type having no light-shading layer. The oil collecting pan has an oil reservoir with an oil outlet opening, and a single scavenging pump for drawing the oil stored in the oil collecting pan through the oil outlet opening. A plurality of different measurements are conducted on consecutive sub-image pair of each hypothesis. A flowmeter for measuring the medium temperature according to the method is also disclosed.