A pattern forming method includes providing a first mask with a first aperture, forming a first transfer pattern on a resist by irradiating a first electron beam through the first aperture, the first transfer pattern extending in a first direction and having a boundary along a circumference thereof, and the first electron beam having a cross section of a first square when emerging from the first aperture, and forming a second transfer pattern on the resist by irradiating a second electron beam through the first aperture, the second transfer pattern extending in the first direction and overlapping a portion the boundary of the first transfer pattern, and the second electron beam having a cross section of a second square when emerging from the first aperture.