Using the Nakagawa's theory of the Logarithmic visus, the author has derived a new method for calculating visibility-factor from visual acuity.The Nakagawa's theory is based on the Hering's idea that “the absolute scale boes not 'exist in the visual space but does only the relative scale”. Assuming that the value of visibility would be in proportion, to the relative decrease of the minimum visual angle and irrelevent to the absolute value, Nakagawa, has deriveb the following formula;dV=-A'dθ/θV=A log-1θ+B=logv+B...(1)Where A and B are constants, θ: minimum visual angle in minutes, v: visual acuity 1/θ V: value indicating the value of visibility.Nakagawa has asserted that the value of visibility must be derived from that formula.Author assumed that A=1 anb B=1 in formula (1), and derived the following formula;Vl=log v+I=log 10v...(2)Where Vl=logarithmic visus indicating the value of visiiblity, v=visual acuity, The relation between v and Vl is shown in Tab. 1.Fig.1 shows that the logarithmic visus is in proportion to the efficiency of the visual task, and Fig.2.7 shows the relation between the visual acuity and the density of fog in comparison of arithmical scale with log, scale of visual acuity. Both data show the fact that the logarithmic visus indicates the value of the visibility.If the logarithmic visus indicates the value of visibility, the rate of decrease in visibility must be calculated with the following equation;log/log-10v'/10v=Vr...(3)Where v=visual acuity of the observer (1/θ).measured with Landoldt's broken ring in the standard condition. Author adopted homogenous illumination of 200±50 lux as the standard condition.v'=visual acuity influenced by the lighting surroundings.Vr=isibility rate indicating the relative visual acuity.Using a 1/25 model of highway tunnel with length 10m and width 0.25m, as shown in Fig.3, illuminated 50lux at the middle and equipped with increased illumination at the entrance, author measured changes in the visual acuities of three observers viewing the tunnel-model from the outside. Results of experiments are shown in Tab. 2 and Fig. 4. Fig.4 shows that the tendency of visiblity for inside of the tunnel appears more clearly in the visibility rate (Vr) than the simple ratiof visual acuity (v'/v).Tab.3 shows another use of the visibility rate, the relation between the visibility rate and the driving safety. As the logarithmic visus is in proportion to the efficiency of the visual task, the safety of a driving car has to be controled by the visibility rate; with this idea, author divided the visibility rate into four sections indicating the safety of driving as shown in Tab. 3.A third use of the visibility rate is for the calculation of the visual acuity influenced by the lighting surroundings, with the following equation;log 10vx=Vr×log 10vWhere v=visual acuity measured in standard conditions.vx=visual acuity influenced by the lighting surroundings.Moreover, author believed that the visibility rate has many other uses as the visibility factor for the estimation of lighting surroundings.