An image forming apparatus includes a holding mechanism that holds an optical writing unit such that, at an operating position, the latent image writing unit is supported at three points with respect to the apparatus main body, and the latent image writing unit and the rotation axis are not in contact with each other. The invention provides for a ramp set suited to be laid out at both sides of the threshold so as to leave the top side thereof uncovered, whereby the door can still be opened and closed. In order to avoid interference, certain ones of the carrier frequencies otherwise used by the broadband powerline communication system may be selectively masked based on the determination. A common supply line supplies a mixture comprising olefins and oxygen to the catalyst.