iMac just turns on if I reset the SMC

My iMac is having a problem to turn on. I always have to disconnect the power cord and reconnect it and then to press the power button. I saw in a search that can be the PRAM's battery, but the system clock is ok. Please, I need help! Sorry for my bad english.

It doesn't apear any screen. It doesn't turn on if I push the power button at a first moment. I need to reset the SMC (disconnect the power cord and reconnect it again) and then, when I push the power button it turns on normally and everything runs well. I turn it off, and on the next day, if I try to turn it on, it fails. I need to reset the SMC again. It just turns on if I reset the SMC.