The present invention is directed to a speaker using lightweight, highly rigid diaphragm having excellent frequency characteristic and improving resistance to environment (including resistance to UV and resistance to fading). For this end, the speaker utilizes a cone-shaped diaphragm made from foamed or cellular resin such as polyethylene terephthalate (PET) resin or polyethylene naphthalate (PEN) resin with 30 .mu.m average cell size. The use of such cone-shaped diaphragm is effective to improve sound quality of the speaker.