An insulated wire having a rectangular shaped cross-section, containing: a conductor having a rectangular shaped cross-section; and an insulating layer coated on the conductor, in which the insulating layer has a foamed layer composed of a thermosetting resin containing bubbles, the cross-section of the insulating layer has a shape composed of a flat portion and a corner portion, and the insulating layer satisfies the relationship represented by the following formula: ∈1 (T1/∈1) wherein T1 [μm] represents a thickness of the flat portion; ∈1 represents a relative dielectric constant of the flat portion; T2 [μm] represents a thickness of the corner portion; and ∈2 represents a relative dielectric constant of the corner portion, and an electric/electronic equipment, a motor, or a transformer, using the insulated wire.