A prototype of thermal neutron beam monitor based on a multiwire proportional chamber design with a thin-film converter B4C has been developed and constructed. The prototype was tested on the 252Cf neutron source, characteristic amplitude spectra were obtained. The possibility of a reliable rejection of unwanted low-amplitude events was demonstrated using the 90Sr β-source. Based on the prototype, two identical neutron beam monitors with the required parameters for the SEM neutron resonance spin-echo spectrometer were manufactured. This instrument is now under construction and will be part of the instrument suite of the high-flux reactor PIK in Gatchina, Russia.