DESCRIPTION TITLE OF INVENTION OPTICAL INFORMATION MEDIUM MEASUREMENT METHOD, OPTICAL INFORMATION MEDIUM, RECORDING APPARATUS, AND REPRODUCING APPARATUS TECHNICAL FIELD [0001] The present invention relates to a measurement method for an optical information medium such as an optical disc. BACKGROUND ART [0002] An optical memory technology that employs an optical disc as a high-density and large- capacity information memory medium has been increasingly applied to a digital audio disc, a video disc, a document file disc, and further, a data file and the like. According to the optical memory technology, information is recorded on the optical disc in a form of a minute pit or a minute record mark. Moreover, the information is recorded and reproduced with high accuracy and high reliability by a small focused light beam. [0003] In a Blu-ray disc (BD) which is one of the optical disc, for example, a minute spot is formed in such a manner that a laser beam having a wavelength in a range from 400 nm to 410 nm, specifically, a wavelength of 405 nm is collected by an objective lens having an NA (Numerical Aperture) in a range from 0.84 to 0.86, specifically, an NA of 0.85. [0004] When the pit or the record mark is reproduced by use of the light beam, a reproduction signal is generated. This reproduction signal needs to have a predetermined property in order to ensure stable reproduction thereof in different apparatuses. Fig. 8A shows an example of such a reproduction signal. As an index for measuring the property of the reproduction signal, adopted is a ratio between an amplitude Ipp of an AC component of the reproduced signal and a signal maximum value Itop, i.e., a modulation degree m (m = Ipp/Itop). This modulation degree m needs to be equal to or more than a specific value to ensure compatibility of optical discs among optical disc devices. For this reason, it is possible to ensure the compatibility of the optical disc among optical disc devices by evaluating the optical disc based on a degree of modulation measured by an optical disc evaluation device (measurement optical system). [0005] In the optical disc having a plurality of recording layers, a degree of modulation m is adversely affected by a reflected light (stray light) from other layers than a target reproduction layer. More specifically, in the case where the stray light is contained in the reproduction signal as shown in FIG. 8B, the signal maximum value lt0p' becomes larger by the stray light as compared to the signal maximum value Itop without the stray light from other layers as shown in FIG. 8A. Consequently, a degree of modulation m2 with an effect of a stray light from other layers than the target reproduction layer is expressed by an equation of m2 = Ipp/Itop', which is disadvantageously smaller as compared to the degree of modulation m without the effect of stray light from other layers than the target reproduction layer. [0006] Like the case of the conventional double-layered disc, when an amount of stray light, which is determined by the area of the light receiving section of the measurement optical system, the magnification of the detection system, and the thickness between layers, is smaller than a predetermined amount, it is possible to ensure the compatibility of optical discs among optical disc devices without problem by setting the degree of modulation m to be not smaller than the predetermined level without taking into consideration such conditions as the area of the light receiving section, the magnification of the detection system, etc. [0007] To ensure stable reproduction among different reproduction apparatuses, a value indicative of the difference in reflectance between layers of a multilayered disc needs to be set within a predetermined range. [0008] Specifically, the difference in reflectance between layers needs to be set in the above range to suppress abrupt changes in signal amplitude when a light beam is moved between the layers or the effect of the stray light from other layers. Namely, in the case where a large difference in reflectance exists between layers, the layer of low reflectance is liable to be affected by large stray light from the layer of higher reflectance, which would significantly affect the degree of modulation of a signal. In contrast, like the conventional double-layered disc, in the case where an amount of stray light, which is determined by the area of the light receiving section of the measurement optical system, the magnification of the detection system, and the thickness between layers, is smaller than a predetermined amount, the compatibility of optical discs among optical disc devices can be ensured without problem by setting the reflectance to fall in the predetermined range without taking such conditions as the area of the light receiving section, the magnification of the detection system, etc. into consideration. [0009] Recently, to increase the recording capacity of the optical disc, practical applications of optica] discs wherein a recording layer is made up of larger number of layers than two layers, such as a triple-layered recording layer, a quadric-layered recording layer, have been made into consideration. For such high density optical disc made up of three or four layers, it is required to reduce the thickness between the layers. With this structure, an amount of stray light that enters into the light receiving section increases as compared to the case of optical discs of double- layered structure, and therefore, the degree of modulation m, or the reflectance would be largely affected by the degree of modulation m or the reflectance would be significantly affected by factors of the optical system such as an area of the light receiving section, the magnification of the detecting system, and the interlayer thickness of the optical disc. Therefore, when adopting a value indicative of the degree of modulation m or a value indicative of the reflectance set in various measurement optical system, a problem arises in that the compatibility of optical discs cannot be ensured among optical disc devices. [0010] As a solution, it may be considered to set predetermined fixed conditions of an optical system for measuring the degree of modulation m or the difference in reflectance, and the degree of modulation m or the difference in reflectance is measured under the fixed conditions as set. However, this countermeasure requires replacement of all optical systems in measuring machines which currently exist in the worldwide, and consequently such solution is far from reality. CITATION LIST PATENT LITERATURE [0011] Patent Literature 1: WO 2007/108507 Al SUMMARY OF INVENTION [0012] It is an object of the present invention to provide an optical information medium measurement method that allows correct comparison of a modulation degree or a difference in reflectance even when an optical information medium is subjected to measurement using any measurement optical system, without preparation of a special measurement optical system. [0013] An optical information medium measurement method according to one aspect of the present invention for measuring a degree of modulation in an optical information medium of a multilayered structure having a plurality of information layers includes: a first step of measuring the modulation degree of each layer of the optical information medium, by use of a measurement optical system, a second step of obtaining a thickness between layers of the optical information medium, a third step of obtaining a reflectance of each layer of the optical information medium, and a fourth step of converting the modulation degree of each layer, the modulation degree being measured in the first step, into a modulation degree for the standard optical system differing from the measurement optical system, based on a value indicative of the thickness between layers, the thickness being obtained in the second step, and a value indicative of the reflectance of each layer, the reflectance being obtained in the third step. [0014] According to the foregoing structure, the comparison of the modulation degree is corrected even when the optical information medium is subjected to measurement using any measurement optical system, without preparing a special measurement optical system. [0015] An optical information medium measurement method according to another aspect of the present invention for measuring a difference in reflectance in an optical information medium of a multilayered structure having a plurality of information layers, includes a fifth step of obtaining an apparent reflectance corresponding to a ratio between a signal light amount and an incident light amount each obtained upon reproduction of information from each layer of the optical information medium, by use of a measurement optical system, a second step of obtaining a thickness between layers of the optical information medium, a third step of obtaining a reflectance of each layer of the optical information medium, and a sixth step of obtaining a result of conversion as a difference in reflectance for the reference optical system differing from the measurement optical system, from a value indicative of the apparent reflectance obtained in the fifth step, a value indicative of the thickness between layers, the thickness being obtained in the second step, and a value indicative of the reflectance of each layer, the reflectance being obtained in the third step. [0016] According to the foregoing structure, the difference in reflectance is corrected even when the optical information medium is subjected to measurement using any measurement optical system, without preparation of a special measurement optical system. [0017] Other objects, characteristics and advantages of the present invention shall be sufficiently clarified by the description herein below. The excellent aspects of the present invention shall be clarified in the following description with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS [0018] Fig. 1A is an explanatory diagram showing a relation between a measurement optical system and stray light from an optical information medium, in accordance with the first embodiment of the present invention. Fig. IB is an explanatory diagram showing a relation between a photodetector of the measurement optical system and the stray light. Fig. 2 is a conceptual diagram showing a triple-layered optical information medium and an optical path, in accordance with the first embodiment of the present invention. Fig. 3 is a conceptual diagram showing a quadric-layered optical information medium and an optical path, in accordance with the first embodiment of the present invention. Fig. 4 is a waveform chart showing an example of a pattern of a measured reproduction signal, in accordance with the first embodiment of the present invention. Fig. 5 is a waveform chart showing an example of a pattern of a reproduction signal converted into a modulation degree at a reference optical system, in accordance with the first embodiment of the present invention. Fig. 6 is a conceptual diagram showing a modulation degree converting method in accordance with the first embodiment of the present invention. Fig. 7 is a conceptual diagram showing a difference in reflectance calculating method in accordance with the second embodiment of the present invention. Fig. 8A is a waveform chart showing an example of a pattern of a reproduction signal without a stray light from other layers. Fig. 8B is a waveform chart showing an example of a pattern of a reproduction signal with a stray light from other layers. Fig. 9 is a conceptual diagram showing a specific example of a structure of the triple- layered optical information medium, in accordance with the first embodiment of the present invention. Fig. 10 is an explanatory diagram showing an optical information medium in accordance with one embodiment of the present invention. Fig. 11 is an explanatory diagram showing a recording apparatus in accordance with one embodiment of the present invention. Fig. 12 is an explanatory diagram showing a reproducing apparatus in accordance with one embodiment of the present invention. Fig. 13 is a conceptual diagram showing one example of a converting method for degree of modulation in accordance with the first embodiment of the present invention. Fig. 14 is a conceptual diagram showing another example of a converting method for degree of modulation in accordance with the first embodiment of the present invention. Fig. 15 is a conceptual diagram showing still another example of a converting method for degree of modulation in accordance with the first embodiment of the present invention. Fig. 16 is a conceptual diagram showing yet another example of a converting method for degree of modulation in accordance with the first embodiment of the present invention. Fig. 17 is a conceptual diagram showing yet another example of a converting method for degree of modulation in accordance with the first embodiment of the present invention. Fig. 18 is a conceptual diagram showing one example of a calculation method for a difference in reflectance in accordance with the second embodiment of the present invention. Fig. 19 is a conceptual diagram showing another example of calculation method for a difference in reflectance in accordance with the second embodiment of the present invention. Fig. 20 is a conceptual diagram showing still another example of a calculating method for a difference in reflectance in accordance with the second embodiment of the present invention. Fig. 21 is a conceptual diagram showing one example of a difference in reflectance calculating method in accordance with the third embodiment of the present invention. DESCRIPTION OF EMBODIMENTS [0019] With reference to the drawings, hereinafter, description will be given of preferred embodiments of the present invention. [0020] (First Embodiment) A method for converting a degree of modulation for each layer of a multilayered optical disc (an optical information medium) as measured by an arbitrary measurement optical system (an optical information medium evaluation apparatus), into a degree of modulation for each layer for the standard optical system in accordance with the first embodiment of the present invention is shown in Fig. 6. This modulation degree converting method includes four steps S201, S202, S203 and S204. The respective steps are described below. Herein, explanations will be given through the case of adopting an optical disc of a triple-layered structure. (S201: Step of measuring a degree of modulation for each layer) [0021] In S201, the degree of modulation is measured for each layer of the optical disc, by the measurement optical system. More specifically, the measurement optical system measures the degree of modulation for each layer, based on a reproduction signal obtained by reproducing information recorded on each layer (e.g., the first layer, the second layer, the third layer) of the optical disc. [0022] Fig. 4 shows an example of the reproduction signal in the measurement optical system. The modulation degree is obtained as a ratio between an amplitude Ipp of an AC component of the reproduction signal and a signal maximum value Itop (IPP/ItoP). A reproduction signal obtained by reproduction of information from a layer subjected to measurement contains a component of stray light reflected from other layers. Accordingly, the degree of modulation of each layer is measured by the measurement optical system, and the measured value contains unique stray light from other layers generated in the measurement optical system. Hereinafter, the measured value for the degree of modulation affected by the stray light from other layers is referred to as an "apparent degree of modulation". [0023] The apparent modulation degrees measured by the measurement optical system, i.e., the apparent modulation degree of the first layer, the apparent modulation degree of the second layer and the apparent modulation degree of the third layer are represented by md], md2 and md3, respectively. [0024] (S202: Step of obtaining thickness between layers) In S202, a thickness between layers of the optical disc is obtained. The thickness between layers of the optical disc may be actually measured using an optical disc to be subjected to measurement and a measuring machine. Moreover, the thickness between layers may take a value such as a design value (a target thickness when manufacturing an optical disc) or an average value of variations upon mass-production of the optical disc (an average thickness in a case of manufacture of a plurality of optical information media). For the thickness between layers, a standard value as specified in accordance with optical disc specifications and the like may be adopted. [0025] (S203: Step of obtaining reflectance of each layer) In S203, a reflectance of each layer of the optical disc is obtained. FIG. 1A which explains S203 shows a schematic structure of the measurement optical system. The measurement optical system is provided with a light source 101, an objective lens 102, a detection lens 104, a photodetector 105 and the like. With reference to Fig. 1A, explanations will be given on a relation between a light receiving part of the measurement optical system and the stray light reflected from other layers. [0026] A light beam emitted from the light source 101 is converged by the objective lens 102 onto the specific information layer (the layer subjected to measurement) of the optical disc 103 (the optical information medium). Light reflected from the optical disc 103 passes through the objective lens 102 again, is collected by the detection lens 104, and enters the photodetector 105, to be converted into an electric signal according to an amount of light. As shown in Fig. IB, the photodetector 105 has the light receiving part 105a. The detecting system of the measurement optical system has a magnification M which is usually obtained from a ratio between a focal distance of the detection lens 104 and a focal distance of the objective lens 102. In Figs. 1A and IB, the stray light reflected from other layers than the layer subjected to measurement is shown with a broken line, for convenience. [0027] An area ratio between an area of extension of the stray light from other layers on the light receiving part 105a and an area of the light receiving part 105a is determined from the detection magnification M of the measurement optical system, the area Sp + Sn2) ... (2-9) [0076] In the case of considering the typical multilayered disc having the "N" layers (2 < N, N: an integer), the difference in reflectance anjj between the i-th layer and the j-th layer is obtained based on the following equation. anfJ =(Sn; -Sn^S^ + 8^) ... (2-10) (1 < i < N, i: an integer) (1 < j i ^ N, i: an integer) is located between the j-th layer (j = i - 1) and the "k"th layer (k = i + 1), the thickness between the i-th layer and the j-th layer, the thickness being obtained in the second step, is represented by dy, the thickness between the i-th layer and the "k"th layer, the thickness being obtained in the second step, is represented by d*, a refractive index between the respective layers is represented by n, a numerical aperture in the measurement optical system is represented by NA, an apparent reflectance corresponding to a ratio of a reflected light amount to an incident light amount upon convergence of light onto the i- th layer in the measurement optical system is represented by Si, and a value 6 satisfies a relation of sin6 = NA/n and falls within a range from 0 to 7i/2, then, in the third step, "N" equations are established so as to satisfy an equation: S; =Rj +Sd-[Rj/{TC(2• du -tane)2} + Rk /{TT(2• d^ -tanG)2}] (in a case of i = 1, Rj/f^-dytane)2} = 0, in a case of i = N, Rk/{7t(2-diktan9)2} = 0), and the reflectance R* is obtained in such a manner that the "N" equations for the apparent reflectance Si are solved with regard to the reflectance R*. 23. The optical information medium measurement method according to any one of claims 16 to 20, wherein the optical information medium is a multilayered disc having "N" layers (2 < N, N: an integer), and when it is assumed that the reflectance of the i-th layer (1 £ i < N, i: an integer) is represented by Ri, and an apparent reflectance corresponding to a ratio of a reflected light amount to an incident light amount upon convergence of light onto the i-th layer in the measurement optical system is represented by Si, then, in the third step, the reflectance Ri approximates to the apparent reflectance S;. 24. The optical information medium measurement method according to claim 1, wherein the optical information medium is a multilayered disc having "N" layers (2 < N, N: an integer), and when it is assumed that the reflectance of the i-th layer (1 < i < N, i: an integer) is represented by Ri, the thickness between the i-th layer and the j-th layer (1 ^ j < N, i * j, j: an integer), the thickness being obtained in the second step, is represented by dy, a refractive index between the i-th layer and the j-th layer is represented by n, a numerical aperture in the measurement optical system is represented by NA, an apparent reflectance corresponding to a ratio of a reflected light amount to an incident light amount upon convergence of light onto the i- th layer in the measurement optical system is represented by Si, and a value 8 satisfies a relation of sin0 = NA/n and falls within a range from 0 to nil, then, in the third step, a conversion coefficient a; is expressed by an equation: &{ =l+Sd-pi/{7i(2-d-tane)2}] (£: addition of integers from 1 to N in a case of j * i, with regard to j), and the reflectance Ri approximates to a relation of Si/a,. 25. The optical information medium measurement method according to claim 16, wherein the optical information medium is a multilayered disc having "N" layers (2 < N, N: an integer), and when it is assumed that the reflectance of the i-th layer (1 < i < N, i: an integer) is represented by Rj, the thickness between the i-th layer and the j-th layer (1 < j < N, i * j, j: an integer), the thickness being obtained in the second step, is represented by dy, a refractive index between the i-th layer and the j-th layer is represented by n, a numerical aperture in the measurement optical system is represented by NA, an apparent reflectance corresponding to a ratio of a reflected light amount to an incident light amount upon convergence of light onto the i- th layer in the measurement optical system is represented by Si, and a value 9 satisfies a relation of sinG = NA/n and falls within a range from 0 to 7i/2, then, in the third step, a conversion coefficient a* is expressed by an equation: a{ = 1 + Sd • [21 /{7t(2 ■ d;j • fan 9)2}] (2: addition of integers from 1 to N in a case of j * i, with regard to j), and the reflectance Rj approximates to a relation of Si/a*. 26. An optical information medium of a- multilayered structure having a plurality of information layers, the optical information medium being subjected to measurement by an optical information medium measurement method comprising: a first step of measuring a degree of modulation of each layer of the optical information medium, by use of a measurement optical system; a second step of obtaining a thickness between layers of the optical information medium; a third step of obtaining a reflectance of each layer of the optical information medium; and a fourth step of converting the modulation degree of each layer, the modulation degree being measured in the first step, into a modulation degree for a standard optical system differing from the measurement optical system, based on a value indicative of the thickness between layers, the thickness being obtained in the second step, and a value indicative of the reflectance of each layer, the reflectance being obtained in the third step. 27. The optical information medium according to claim 26, wherein: in the third step, the reflectance of each layer is obtained by using an apparent reflectance, the apparent reflectance being a reflectance to be measured including an influence of stray light reflected from a layer other than a layer to be measured. 28. The optical information medium according to claim 26, wherein: in the fourth step, the modulation degree of each layer measured in the first step is further converted into a modulation degree for the standard optical system differing from the measurement optical system by using an apparent reflectance, the apparent reflectance being a reflectance to be measured including an influence of stray light reflected from a layer other than a layer to be measured. 29. The optical information medium according to claim 26, wherein: the optical information medium is a multilayered disc having "N" layers (2 < N, N: an integer), a modulation degree of the i-th layer (1 < i < N, i: an integer), the modulation degree being measured in the first step, is represented by mdj, a thickness between the i-th layer and the j-th layer (1 ^ j < N, i * j, j: an integer), the thickness being obtained in the second step, is represented by dy, a refractive index between the i-th layer and the j-th layer is represented by n, an apparent reflectance corresponding to a ratio of a reflected light amount to an incident light amount upon convergence of light onto the i-th layer in the measurement optical system is represented by Si, the reflectance of the i-th layer (1 < i < N, i: an integer), the reflectance being obtained in the third step, is represented by Rj, a value 9 satisfies a relation of sinG = NA/n and falls within a range from 0 to rc/2, an area of a light receiving part in the standard optical system is represented by Snpd, a magnification of a detecting system in the standard optical system is represented by Mn, a normalize light receiving part size in the standard optical system is represented by Sdn (Sdn = Snpd/Mn2), and in the fourth step, the modulation degree mnj of the i-th layer for the standard optical system is expressed by an equation: mn, =mdt •Si l{Rt + Sdn[LRj l{x{2-dtj -tan0)2}]) (S: addition of integers from 1 to N in a case of j *■ i, with regard to j). 30. A recording apparatus for recording information on the optical information medium according to any one of claims 26 to 29, wherein the information is recorded by irradiating the optical information medium with a light beam. 31. A reproducing apparatus for reproducing information from the optical information medium according to any one of claims 26 to 29, wherein the information is reproduced by irradiating the optical information medium with a light beam. 32. A recording apparatus for measuring a degree of modulation in an optical information medium of a multilayered structure having a plurality of information layers, and for recording information on the optical information medium, wherein: the modulation degree of each layer of the optical information medium is measured by a measurement optical system included in the recording apparatus, a thickness between layers of the optical information medium is obtained, a reflectance of each layer of the optical information medium is obtained, the measured modulation degree of each layer is converted into a modulation degree for a standard optical system differing from the measurement optical system, based on a value indicative of the thickness between layers and a value indicative of the reflectance of each layer, and information is recorded by irradiating the optical information medium with a light beam. 33. The recording apparatus according to claim 32, wherein: in obtaining the reflectance of each layer, the reflectance of each layer is obtained by using an apparent reflectance, the apparent reflectance being a reflectance to be measured including an influence of stray light reflected from a layer other than a layer to be measured. 34. The recording apparatus according to claim 32, wherein: in converting into the modulation degree for the standard optical system differing from the measurement optical system, the modulation degree of each layer measured is further converted into the modulation degree for the standard optical system differing from the measurement optical system by using an apparent reflectance, the apparent reflectance being a reflectance to be measured including an influence of stray light reflected from a layer other than a layer to be measured. 35. The recording apparatus according to claim 32, wherein: the optical information medium is a multilayered disc having "N" layers (2 < N, N: an integer), a modulation degree of the i-th layer (1 < i < N, i: an integer), the modulation degree being measured in measuring the modulation degree of each layer, is represented by md,, a thickness between the i-th layer and the j-th layer (1 ^ j < N, i * j, j:an integer), the thickness being obtained in obtaining the thickness between layers, is represented by dy, a refractive index between the i-th layer and the j-th layer is represented by n, an apparent reflectance corresponding to a ratio of a reflected light amount to an incident light amount upon convergence of light onto the i-th layer in the measurement optical system is represented by Si, the reflectance of the i-th layer (1 ^ i < N, i: an integer), the reflectance being obtained in obtaining the reflectance of each layer, is represented by Ri, a value 0 satisfies a relation of sinG = NA/n and falls within a range from 0 to Till, an area of a light receiving part in the standard optical system is represented by Snpd, a magnification of a detecting system in the standard optical system is represented by Mn, a normalize light receiving part size in the standard optical system is represented by Sdn (Sdn = Snpd/Mn2), and the modulation degree mrii of the i-th layer for the standard optical system is expressed by an equation in converting into the modulation degree for the standard optical system differing from the measurement optical system: mn, = md, -S, l(R, +Sdn-\LRj /{x(2-dtj -tanfl)2}]) (2: addition of integers from 1 to N in a case of j * i, with regard to j). 36. A reproducing apparatus for measuring a degree of modulation in an optical information medium of a multilayered structure having a plurality of information layers, and for reproducing information from the optical information medium, wherein the modulation degree of each layer of the optical information medium is measured by a measurement optical system included in the reproducing apparatus, a thickness between layers of the optical information medium is obtained, a reflectance of each layer of the optical information medium is obtained, the measured modulation degree of each layer is converted into a modulation degree for a standard optical system differing from the measurement optical system, based on a value indicative of the thickness between layers and a value indicative of the reflectance of each layer, and information is reproduced by irradiating the optical information medium with a light beam. 37. The reproducing apparatus according to claim 36, wherein: in obtaining the reflectance of each layer, the reflectance of each layer is obtained by using an apparent reflectance, the apparent reflectance being a reflectance to be measured including an influence of stray light reflected from a layer other than a layer to be measured. 38. The reproducing apparatus according to claim 36, wherein: in converting into the modulation degree for the standard optical system differing from the measurement optical system, the modulation degree of each layer measured is further converted into the modulation degree for the standard optical system differing from the measurement optical system by using an apparent reflectance, the apparent reflectance being a reflectance to be measured including an influence of stray light reflected from a layer other than a layer to be measured. 39. The reproducing apparatus according to claim 36, wherein: the optical information medium is a muluTayered disc having "N" layers (2 < N, N: an integer), a modulation degree of the i-th layer (1 < i < N, i: an integer), the modulation degree being measured in measuring the modulation degree of each layer, is represented by mdi, a thickness between the i-th layer and the j-th layer (1 < j < N, i * j, j: an integer), the thickness being obtained in obtaining the thickness between layers, is represented by d„, a refractive index between the i-th layer and the j-th layer is represented by n, an apparent reflectance corresponding to a ratio of a reflected light amount to an incident light amount upon convergence of light onto the i-th layer in the measurement optical system is represented by Si, the reflectance of the i-th layer (1 < i < N, i: an integer), the reflectance being obtained in obtaining the reflectance of each layer, is represented by Ri, a value 0 satisfies a relation of sinG = NA/n and falls within a range from 0 to nil, an area of a light receiving part in the standard optical system is represented by Snpd, a magnification of a detecting system in the standard optical system is represented by Mn, a normalize light receiving part size in the standard optical system is represented by Sdn (Sdn = Snpd/Mn2), and in converting into the modulation degree for the standard optical system differing from the measurement optical system, the modulation degree mnj of the i-th layer for the standard optical system is expressed by an equation: mn, = mdt • St /(R, + Sdn ■ \LRj l{n(2 • dtj • tan &?}]) (2: addition of integers from 1 to N in a case of j * i, with regard to j). 40. An optical information medium measurement method for measuring a reflectance in an optical information medium of a multilayered structure having "N" information layers (2 < N, N: an integer), comprising: when it is assumed that the reflectance of the i-th layer (1 < i ^ N, i: an integer) is represented by R,, the thickness between the i-th layer and the j-th layer (1 < j < N, i *■ j, j: an integer) is represented by dij, a refractive index between the i-th layer and the j-th layer is represented by n, an apparent reflectance corresponding to a ratio of a reflected light amount to an incident light amount upon convergence of light onto the i-th layer in the measurement optical system is represented by Si, a numerical aperture in the measurement optical system is represented by NA, and a value 0 satisfies a relation of sin0 = NA/n and falls within a range from 0 to 7t/2, "N" equations are established so as to satisfy an equation: Sj = Rj + Sd • [ZR ■ /{7r(2 • dij • tan Q)2}] (2: addition of integers from 1 to N in a case of j * i, with regard to j), and the reflectance R, is obtained in such a manner that the "N" equations for the apparent reflectance Si are solved with regard to the reflectance R,. 41. An optical information medium to be measured by the optical information medium measurement method of claim 40. 42. A recording apparatus for recording information on the optical information medium of claim 41, wherein the information is recorded by irradiating the optical information medium with a light beam. 43. A reproducing apparatus for reproducing information from the optical information medium of claim 41, wherein the information is reproduced by irradiating the optical information medium with a light beam. Provided is an optical informa- tion medium measuring method for measuring the modulation degree of an optical informa- tion medium which has a multilayer structure having a plurality of information layers. The method includes: a first step of measuring the modulation degree of each layer of the optical information medium by a measuring optical system; a second step of obtaining the thick- ness between the layers of the optical informa- tion medium; a third step of obtaining the re- flectance of each layer of the optical informa- tion medium; and a fourth step of converting the modulation degree of each layer measured in the first step into a modulation degree in a reference optical system different from the measuring optical system by using the thick- nesses between the layers obtained in the sec- ond step and the reflectance of each layer ob- tained in the third step.