A flow rate diagnosis apparatus includes: a main line in which a tank having a volume is provided; a branch line that branches from the main line on an upstream side of the tank; a first open close valve provided in the branch line; a second open close valve provided in the main line; a dead volume defined from the diagnosis object as an upstream end and the first open close valve and the second open close valve as downstream ends; and a second pressure control mechanism that controls a fluid flowing through the main line such that the pressure of the fluid in the dead volume is maintained at a second setting pressure during an inflow mode during which the fluid is caused to flow into the tank by closing of the first open close valve and opening of the second open close valve after a preparation mode.