[en] W-values (average energy loss per ion pair) were measured on four kinds of tissue equivalent gases for α-particles from ²⁴¹Am by a modified Jesse method. A chart recorder following an electrometer figuring in Jesse's apparatus was replaced by a 'peak holder' circuit and a pulse height analyzer, which made it possible to measure the pulse height distribution directly. The measured W-values were 31.1 eV for Rossi's TE gas, 28.5 eV for Shonka's TE gas, 27.2 eV for propane-butane-air mixture and 27.1 eV for a new TE gas. Strickler's formula which has been used to predict W-values of binary gas mixtures is modified into a formula applicable to the mixture of three gases or more. The estimated W-values of Rossi's TE gas and propane-butane-air mixture from the modified formula were in excellent agreement with the measured results. (author)