A measuring apparatus for measuring flow, such as liquid or gas flow, and/or its characteristics. The apparatus comprises a housing (1) provided with a flow channel (2) and a tubular member fitted thereinto, and inside the tubular member there are coaxially installed a flow control member (12) and thereafter, in the flowing direction, a rotor (10), or a combination of the two. The housing (1) is provided at least with a rotor revmeter (3) and connecting means for connecting the measuring apparatus into the flow circuit. According to the invention, the tubular member is formed as a replacement piece (7) which is provided with members (10, 12) installed thereinto. The replacement piece can be alternatively replaced according to the needs, for instance in the case of malfunction, or according to the specific measuring purposes, either by a new replacement piece or by another, different replacement piece, whereinto there are installed at least the said flow control member and the rotor, or a combination of the two.