The present invention provides a diaphragm for a loudspeaker which suppresses divided resonance and shows a stable sound-pressure-frequency characteristic, and a loudspeaker using the diaphragm. The diaphragm includes three or more thick parts (11c) of odd numbers formed radially from a center part to an outer periphery, and semi thick part (11d) formed between the thick parts so as to become thinner gradually from the outer periphery to the center part. Web shaped thin part (11e) is formed at an inner part of the semi thick part.