A series of experiments was carried out by using the Kyoto University Critical Assembly and a pulsed neutron generator attached. The experiments were intended to investigate a variance-to-mean technique for measuring the prompt neutron decay constant alpha of the subcritical reactor system that was driven by the periodic and pulsed neutron source. As the results, it was confirmed that the a value measured by the present technique agreed with a reference value evaluated by the pulsed neutron technique. Furthermore, it was demonstrated that the present technique was available to measure the a value even when the product of a and the repetition period of pulsed neutrons was smaller than unity. Since measurement of alpha by the pulsed neutron technique is generally difficult under such conditions, it was concluded that the present technique was convenient for measuring the alpha value. (C) 2003 Elsevier Ltd. All rights reserved.