In this paper, noise in resistive wall amplifier – vacuum microwave device is studied, in which in which the gain is due to the phase shift between the electron beam and the alternating field components due to the presence of absorbing walls. It should be noted that in resistive wall amplifier, there is no need for a slowing system and that almost completely there is completely lacks the feedback between the output and the input. Recently, attention has been paid to it, in particular, because of the possibility of using metamaterials that increase the gain factor. In early works, on the basis of experimental data, it was mentioned that resistive wall amplifier has rather high noises, but no theoretical estimate has been made. The linear theory of a device with an infinitely wide electronic flow is considered when only an increasing wave is taken into account. An expression for the gain was obtained and an estimate of the noise factor was made assuming that the lamp has a resonant input and output device and that the noise is caused by uncorrelated current and velocity fluctuations in the absence of a minimum potential. In the present work, the theoretical estimation of the noise factor of the amplifier for absorption was carried out for the first time. It is shown that with the parameters considered, the gain can reach more than 20 dB for a resistor section length equal to 30 cm, and the noise factor reaches 24 dB. With the assumptions introduced in this paper and the typical parameters of the cathode and resonators taken to calculate, a rather large noise factor is obtained at a sufficiently high gain.