Provided is an optical module which can be efficiently manufactured. The optical module of the present invention comprises a housing having a lower casing, an upper casing, and a cover; an optical sub-assembly; a circuit board; and a block. The lower casing includes a receptacle mating with an optical connector, and a mount. The upper casing engages with the lower casing. The cover covers the upper casing. The optical sub-assembly optically couples with the optical connector in the receptacle. The circuit board is electrically connected to the optical sub-assembly and mounted to the mount of the lower casing. The block is mounted to the lower casing, and defines relative positions of the lower casing, upper casing, optical sub-assembly, and circuit board.