A method of measuring inter-core crosstalk of a multi-core optical fiber , the method comprising the steps of: disposing, at a position where light outputted from the multi-core optical fiber reaches, a light-receiving waveguide (50; 70) which guides light outputted from one or more cores of the multi-core optical fiber , causing light to enter at least a first core on a first end face of the multi-core optical fiber ; guiding light outputted from at least a second core on a second end face opposing the first end face of the multi-core optical fiber through the light-receiving waveguide (50; 70), the second core being different from the first core ; receiving the light having been guided through the light-receiving waveguide (50; 70), and measuring the light having been guided through the light-receiving waveguide (50; 70) with a power meter; characterized in that : the light-receiving waveguide (50; 70) has one or more cores and a cladding (A), and a refractive index structure of depressed type or trench-assisted type in which each core (B) is surrounded by a layer (C; D) having a refractive index lower than that of the cladding (A), the layer (C; D) constituting the depressed type or the trench-assisted type is positioned inside the cladding (A) or between the cladding (A) and the core (B), and the layer (C; D) constituting the depressed type or the trench-assisted type is constituted by only solid having a refractive index lower than that of the cladding (A), or formed as a layer in which an average refractive index thereof is lower than that of the cladding (A) as a result of having holes in the solid. disposing, at a position where light outputted from the multi-core optical fiber reaches, a light-receiving waveguide (50; 70) which guides light outputted from one or more cores of the multi-core optical fiber , causing light to enter at least a first core on a first end face of the multi-core optical fiber ; guiding light outputted from at least a second core on a second end face opposing the first end face of the multi-core optical fiber through the light-receiving waveguide (50; 70), the second core being different from the first core ; receiving the light having been guided through the light-receiving waveguide (50; 70), and measuring the light having been guided through the light-receiving waveguide (50; 70) with a power meter; characterized in that : the light-receiving waveguide (50; 70) has one or more cores and a cladding (A), and a refractive index structure of depressed type or trench-assisted type in which each core (B) is surrounded by a layer (C; D) having a refractive index lower than that of the cladding (A), the layer (C; D) constituting the depressed type or the trench-assisted type is positioned inside the cladding (A) or between the cladding (A) and the core (B), and the layer (C; D) constituting the depressed type or the trench-assisted type is constituted by only solid having a refractive index lower than that of the cladding (A), or formed as a layer in which an average refractive index thereof is lower than that of the cladding (A) as a result of having holes in the solid. the light-receiving waveguide (50; 70) has one or more cores and a cladding (A), and a refractive index structure of depressed type or trench-assisted type in which each core (B) is surrounded by a layer (C; D) having a refractive index lower than that of the cladding (A), the layer (C; D) constituting the depressed type or the trench-assisted type is positioned inside the cladding (A) or between the cladding (A) and the core (B), and the layer (C; D) constituting the depressed type or the trench-assisted type is constituted by only solid having a refractive index lower than that of the cladding (A), or formed as a layer in which an average refractive index thereof is lower than that of the cladding (A) as a result of having holes in the solid. <EOS>