The invention relates to a method of measuring a stress wave and to a measuring device and a rock breaking device. A percussion device (7) gives impact pulses to a waveguide (21), where a compression stress wave and a reflected tensile stress wave are generated, which propagate in the waveguide. The compression stress wave causes an extension in the waveguide and the tensile stress wave a thinning, in which case properties of the waveguide may be determined by measuring geometric changes in the cross section of the waveguide. The measurement data are utilized in controlling the rock breaking device.