A transformer comprising: a winding member, a core member being made of magnetic material, said core member being disposed around said winding member such that said core member forms a magnetic path, wherein said winding member comprises at least two second substrates (2, 4) and a first substrate (20; 1, 3; 22, 23, 24) which are stacked, each one of said first substrate (20; 1, 3; 22, 23, 34) and said second substrates (2, 4) is provided with a respective insertion hole (1a-4a, 20a) and a respective patterned conductor (7-10, 21, 26-28) which surrounds said insertion hole (1a-4a, 20a), said insertion hole (1a-4a, 20a) is provided for passing a middle leg portion of said core member, said patterned conductor (21; 7, 9; 26, 27, 28) of said first substrate (20; 1, 3; 22, 23, 24) being defined by a one-turn patterned conductor which has one turn, the patterned conductor having ends defining an end side of said one-turn patterned conductor, whereby said patterned conductor (21; 7, 9; 26, 27, 28) of said first substrate (20; 1, 3; 22, 23, 24) defines a first winding, said patterned conductor (8, 10) of each at least two said second substrates (2, 4) defines a second winding, wherein said transformer further comprises interlayer connection members (16, 17) which are configured to respectively electrically connect said patterned conductors (8, 10) of said at least two second substrates (2, 4), which are stacked, to each other, said interlayer connection members (16, 17) being located in an end side of said one-turn patterned conductor (7, 9, 21, 26-28) and inward of said one-turn patterned conductor (7, 9, 21, 26-28), characterized in that a distance between said interlayer connection member (16, 17) and said middle leg portion is equal to or more than 0.5 mm. a winding member, a core member being made of magnetic material, said core member being disposed around said winding member such that said core member forms a magnetic path, wherein said winding member comprises at least two second substrates (2, 4) and a first substrate (20; 1, 3; 22, 23, 24) which are stacked, each one of said first substrate (20; 1, 3; 22, 23, 34) and said second substrates (2, 4) is provided with a respective insertion hole (1a-4a, 20a) and a respective patterned conductor (7-10, 21, 26-28) which surrounds said insertion hole (1a-4a, 20a), said insertion hole (1a-4a, 20a) is provided for passing a middle leg portion of said core member, said patterned conductor (21; 7, 9; 26, 27, 28) of said first substrate (20; 1, 3; 22, 23, 24) being defined by a one-turn patterned conductor which has one turn, the patterned conductor having ends defining an end side of said one-turn patterned conductor, whereby said patterned conductor (21; 7, 9; 26, 27, 28) of said first substrate (20; 1, 3; 22, 23, 24) defines a first winding, said patterned conductor (8, 10) of each at least two said second substrates (2, 4) defines a second winding, wherein said transformer further comprises interlayer connection members (16, 17) which are configured to respectively electrically connect said patterned conductors (8, 10) of said at least two second substrates (2, 4), which are stacked, to each other, said interlayer connection members (16, 17) being located in an end side of said one-turn patterned conductor (7, 9, 21, 26-28) and inward of said one-turn patterned conductor (7, 9, 21, 26-28), characterized in that a distance between said interlayer connection member (16, 17) and said middle leg portion is equal to or more than 0.5 mm.