A prototype of a device intended for express-measurements of water isotopic composition (Н217O, H217O, H218O, HD16O, D216O) by diode laser spectroscopy (DLS) methods is proposed for the first time. A diode laser (DL) with a distributed feedback (product of "Laser Components") with λ = 1.39 μm was used in the measurements. The DL was pumped by trapezoidal pulses with a pulse length of 0.5-1 ms, a pulse repetition frequency of 1-2 kHz, and tuning range in a mode of 1.5 cm-1 (7183.2-7185 cm--1). These parameters allowed a simultaneous determination of concentration of five water isotopomers. Real-time detection and processing of absorption spectra for water isotopes were conducted with the use of a National Instruments data acquisition board and LabVIEW software. A single measurement of isotopic ratio factor took less than 3-5 min. The range of H2O isotope measured concentrations made 0.01-100%.