Abstract: An instrument for measuring the peak flow during an exhalation, which can be quickly and simply assembled and disassembled manually, so that a simple and cost effective production is possible, on the one hand, and a complete cleaning of the instrument is possible on the other hand, so that a hygienic use is guaranteed. The case of the instrument consists of two halves (12 and 22). The first half (12) of the case is equipped with a projection (18), which locks into a recess (24) in the second half (22) of the case when the case is assembled. The two halves of the case (12 and 22) are furthermore both equipped with an extension piece (22 or 26, respectively), with the two extension pieces resting on each other when the case is assembled. For the assembly, the projection (18) installed on the second half (22) of the case is rotated into the recess (24) in the second half of the case, so that the two halves (12 and 22) of the case are joined and the two extension pieces (20 and 26) rest on each other.